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Older Driver Highway Design Handbook

I. INTERSECTIONS (AT-GRADE)

1. Intersecting Angle (Skew)
2. Receiving Lane (Throat)
   Width for Turning Operations
3. Channelization
4. Intersection Sight Distance
   (Sight Triangle)
5. Opposite (Single) Left-Turn Lane Geometry, Signing, and Delineation
6. Edge Treatments/Delineation of Curbs, Medians, and Obstacles
7. Curb Radius
8. Traffic Control for Left-Turn Movements at Signalized Intersections
9. Traffic Control for Right-Turn/RTOR Movements at Signalized Intersections
10. Street-Name Signage
11. One-Way/Wrong-Way Signage
12. Stop- and Yield-Controlled
    Intersection Signage
13. Devices for Lane Assignment on Intersection Approach
14. Traffic Signal Performance Issues
15. Fixed Lighting Installations
16. Pedestrian Control Devices

A. Design Element: Intersecting Angle (Skew)

Table 1. Cross-references of related entries for intersecting angle (skew).

Skewed intersections pose particular problems for older drivers. Many older drivers experience a decline in head and neck mobility, which accompanies advancing age and may contribute to the slowing of psychomotor responses. Joint flexibility, an essential component of driving skill, has been estimated to decline by approximately 25 percent in older adults due to arthritis, calcification of cartilage, and joint deterioration (Smith and Sethi, 1975). A restricted range of motion reduces an older driver's ability to effectively scan to the rear and sides of his or her vehicle to observe blind spots, and similarly may be expected to hinder the timely recognition of conflicts during turning and merging maneuvers at intersections (Ostrow, Shaffron, and McPherson, 1992). For older drivers, diminished physical capabilities may affect their performance at intersections designed with acute angles by requiring them to turn their heads further than would be required at a right-angle intersection. This obviously creates more of a problem in determining appropriate gaps. For older pedestrians, the longer exposure time within the intersection becomes a major concern.

In a survey of older drivers conducted by Yee (1985), 35 percent of the respondents reported problems with arthritis and 21 percent indicated difficulty in turning their heads to scan rearward while driving. Excluding vision/visibility problems associated with nighttime operations, difficulty with head turning placed first among all concerns mentioned by older drivers participating in a more recent focus group conducted to examine problems in the use of intersections where the approach leg meets the main road at a skewed angle, and/or where channelized right-turn lanes require an exaggerated degree of head/neck rotation to check for traffic conflicts before merging (Staplin, Harkey, Lococo, and Tarawneh, 1997). Comments about this geometry centered around the difficulty older drivers experience turning their heads at angles less than 90 degrees to view traffic on the intersecting roadway, and several participants reported an increasing reliance on outside rearview mirrors when negotiating highly skewed angles. However, they reported that the outside mirror is of no help when the roads meet at the middle angles (e.g., 40 to 55 degrees) and a driver is not flexible enough to physically turn to look for traffic. In an observational field study conducted as a part of the same project, Staplin et al. (1997) found that approximately 30 percent of young/middle-aged drivers (ages 25­45) and young-old drivers (ages 65­74) used their mirrors in addition to making head checks before performing a right-turn-on-red (RTOR) maneuver at a skewed intersection (a channelized right-turn lane at a 65-degree skew). By comparison, none of the drivers age 75 and older used their mirrors; instead, they relied solely on information obtained from head/neck checks. In this same study, it was found that the likelihood of a driver making an RTOR maneuver is reduced by intersection skew angles that make it more difficult for the driver to view conflicting traffic.

The practical consequences of restricted head and neck movement on driving performance at T-intersections were investigated by Hunter-Zaworski (1990), using a simulator to present videorecorded scenes of intersections with various levels of traffic volume and sight distance in a 180-degree field of view from the driver's perspective. Drivers in two subject groups, ages 30­50 and 60­80, depressed a brake pedal to watch a video presentation (on three screens), then released the pedal when it was judged safe to make a left turn; half of each age group had a restricted range of neck movement as determined by goniometric measures of maximum (static) head-turn angle. Aside from demonstrating that skewed intersections are hazardous for any driver with an impairment in neck movement, this study found that maneuver decision time increased with both age and level of impairment. Thus, the younger drivers in this study were able to compensate for their impairments, but older drivers both with and without impairments were unable to make compensations in their (simulated) intersection response selections.

These research findings reinforce the desirability of providing a 90-degree intersection geometry and endorse the ITE (1984) recommendation establishing a 75-degree minimum as a practice to accommodate age-related performance deficits.

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B. Design Element: Receiving Lane (Throat) Width for Turning Operations

Table 2. Cross-references of related entries for receiving lane (throat) width for turning operations.

Design recommendations for lane width at intersections follow from consideration of vehicle maneuver requirements and their demands on drivers. Positioning a vehicle within the lane in preparation for turning has been rated as a critical task (McKnight and Adams, 1970). Swinging too wide to lengthen the turning radius and minimize rotation of the steering wheel ("buttonhook turn") is a common practice of drivers lacking strength (including older drivers) and physically limited drivers (McKnight and Stewart, 1990).

Two factors can compromise the ability of older drivers to remain within the boundaries of their assigned lanes during a left turn. One factor is the diminishing ability to share attention (i.e., to assimilate and concurrently process multiple sources of information from the driving environment). The other factor involves the ability to turn the steering wheel sharply enough, given the speed at which they are traveling, to remain within the boundaries of their lanes. Some older drivers seek to increase their turning radii by initiating the turn early and rounding-off the turn. The result is either to cut across the apex of the turn, conflicting with vehicles approaching from the left, or to intrude upon a far lane in completing the turn.

The Intersection Channelization Design Guide (Neuman, 1985) states that "left-turn lane widths should reflect the speed, volume, and vehicle mix. Although 3.6-m (12-ft) widths are desirable, lesser widths may function effectively and safely. Absolute minimum widths of 2.7 m (9 ft) should be used only in unusual circumstances, and only on low-speed streets with minor truck volumes." Similarly, the ITE (1984) guidelines suggest a minimum lane width of 3.3 m (11 ft) and specify 3.6 m (12 ft) as desirable. These guidelines suggest that wider lanes be avoided due to the resulting increase in pedestrian crossing distances. However, the ITE guidelines provide a range of lane widths at intersections from 2.7 m to 4.3 m (9 ft to 14 ft), where the wider lanes would be used to accommodate larger turning vehicles, which have turning paths that sweep a path from 4.1 m (13.6 ft) for a single-unit truck or bus, up to 6.3 m (20.6 ft) for a semitrailer.

Results of field observation studies conducted by Firestine, Hughes, and Natelson (1989) found that trucks performing turns on urban roads encroached into other lanes on streets with widths of less than 3.6 m (12 ft). They noted that on rural roads, lanes wider than 3.6 m or 4.0 m (12 ft or 13 ft) allowed oncoming vehicles to move further right to avoid trucks, and shoulders wider than 1.2 m (4 ft) allowed oncoming vehicles a greater margin of safety.

In an observational field study conducted to determine how older drivers (age 65 and older) compare with younger drivers during left-turn operations under varying intersection geometries, one variable that showed significant differences in older and younger driver behavior was turning path (Staplin, Harkey, Lococo, and Tarawneh, 1997). Older drivers encroached into the opposing lane of the cross street (see figure 1, turning path trajectory number 1) when making the left turn more often than younger drivers at the location where the throat width (equivalent to the lane width) measured 3.6 m (12 ft). At the location where the throat width measured 7 m (23 ft), which consisted of a 3.6-m (12-ft) lane and a 3.3-m (11-ft) shoulder, there was no significant difference in the turning paths. The narrower throat width resulted in higher encroachments by older drivers, who physically may have a harder time maneuvering their vehicles through smaller areas.

These data sources indicate that a 3.6-m (12-ft) lane width provides the most reasonable tradeoff between the need to accommodate older drivers, as well as larger turning vehicles, without penalizing the older pedestrian in terms of exaggerated crossing distance.

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C. Design Element: Channelization

Table 3. Cross-references of related entries for channelization.

The spatial visual functions of acuity and contrast sensitivity are important in the ability to detect/recognize downstream geometric features such as pavement width transitions, channelized turning lanes, island and median features across the intersection, and any nonreflectorized raised elements at intersections. Visual acuity (the ability to see high-contrast, high-spatial-frequency stimuli, such as black letters on a white eye chart) shows a slow decline beginning at approximately age 40, and marked acceleration at age 60 (Richards, 1972). Approximately 10 percent of men and women between ages 65 and 75 have acuity worse than 20/30, compared with roughly 30 percent over the age of 75 (Kahn, Leibowitz, Ganley, Kini, Colton, Nickerson, and Dawber, 1977). A driver's response to intersection geometric features is influenced in part by the processing of high-spatial-frequency cues—for example, the characters on upstream advisory signs—but it is the larger, often diffuse edges defining lane and pavement boundaries, curb lines, and raised median barriers that are the targets with the highest priority of detection for safety. Older persons' sensitivity to visual contrast (the ability to see objects of various shapes and sizes under varying levels of contrast) also declines beginning around age 40, then declines steadily as age increases (Owsley, Sekuler, and Siemsen, 1983). Poor contrast sensitivity has been shown to relate to increased crash involvement for drivers age 66 and older, when incorporated into a battery of vision tests also including visual acuity and horizontal visual field size (Decina and Staplin, 1993).

The effectiveness of channelization from a safety perspective has been documented in several studies. An evaluation of Federal Highway Safety Program projects showed channelization to produce an average benefit/cost ratio of 2.31 (Strate, 1980). One of the advantages of using curbed medians and intersection channelization is that it gives a better indication to motorists of the proper use of travel lanes at intersections. In a set of studies performed by the California Department of Public Works investigating the differences in accident experience with raised versus painted channelization, the findings were as follows: raised traffic islands are more effective than painted islands in reducing frequencies of night accidents, particularly in urban areas; and little difference is noted in the effectiveness of raised versus painted channelizing islands at rural intersections (Neuman, 1985).

One of the most common uses of channelization is for the separation of left-turning vehicles from the through-traffic stream. The reasons for designing intersections with left-turn lanes include: (1) proven safety effectiveness, (2) effectiveness in improving intersection capacity, (3) flexibility in possible signal phasing schemes, and (4) better understanding of intended traffic operations by the driving public. Guidance on when to include left-turn lanes varies with each State, as revealed in a survey of practices conducted by Neuman (1985).

The safety benefits of left-turn channelization have been documented in several studies. A study by McFarland, Griffin, Rollins, Stockton, Phillips, and Dudek (1979) showed that accidents at signalized intersections where a left-turn lane was added, in combination with and without a left-turn signal phase, were reduced by 36 percent and 15 percent, respectively. At nonsignalized intersections with painted channelization separating the left-turn lane from the through lane, accidents were reduced for rural, suburban, and urban areas by 50, 30, and 15 percent, respectively. When raised channelization devices were used, the accident reductions were 60, 65, and 70 percent in rural, suburban, and urban areas, respectively. Hagenauer, Upchurch, Warren, and Rosenbaum (1982) found that the channelization of intersections reduced accidents by 32 percent and injury accidents by 50 percent.

On the other hand, it was reported in Transportation Research Circular 382 (Transportation Research Board, 1991) that the aging driver, having poorer vision, slower physical reaction time, lower degree of awareness, and reduced ability to maneuver the vehicle, is more likely to be negatively affected by a raised median than is the average driver; and because medians are fixed objects, when they are struck they pose a serious threat of loss of control, especially for aging drivers. The typical curbed median offers low to no contrast with the adjacent pavement and is difficult to reflectorize at night. Low-beam headlight limitations, coupled with reduced vision of the aging driver, compounds the visibility problem. In addition, raised medians and raised corner islands, when used together, often create turning path options at complex intersections that are confusing to the average driver, and disproportionately so for the aging one. Thus, to realize the safety benefits channelization can provide, it is particularly important to ensure the visibility of raised surfaces for (older) drivers with diminished vision, so these road users can detect the channelizing devices and select their paths accordingly.

Another benefit in the use of channelization is the provision of a refuge for pedestrians. Refuge islands are a design element that can aid older pedestrians who have slow walking speeds. While the intent and purpose of the refuge island is well defined, no quantitative warrants are provided by either the MUTCD or AASHTO to determine when such an island is needed. However, areas where they are likely to be needed (e.g., multilane roadways and large or irregularly shaped intersections) are identified in both documents. Once the need is determined, the size and location of such islands can be determined with the help of these two documents. Also quite useful as a reference in this area is Accessibility for Elderly and Handicapped Pedestrians—A Manual for Cities (Earnhart and Simon, 1987).

With respect to the Hagenauer et al. (1982) study cited earlier, Hauer (1988) stated that because channelization in general serves to simplify an otherwise ambiguous and complex situation, the channelization of an existing intersection might enhance both the safety and mobility of older persons, as well as enhance the safety of other pedestrians and drivers. However, in designing a new intersection, he stated that the presence of islands is unlikely to offset the disadvantage of large intersection size for the pedestrian.

Staplin, Harkey, Lococo, and Tarawneh (1997) conducted a field study evaluating four right-turn lane geometries to examine the effect of channelized right-turn lanes and the presence of skew on right-turn maneuvers made by drivers of different ages. One hundred subjects divided across three age groups drove their own vehicles around test routes using the local street network in Arlington, VA. The three age groups were young/middle-aged (ages 25­45), young-old (ages 65­74), and old-old (age 75 and older). As diagrammed in figure 2, the four right-turn lane geometries were:

(a) A nonchannelized 90-degree intersection where drivers had the chance to make a right turn on red (RTOR) around a 12.2-m (40-ft) radius. This site served as a control geometry to examine how channelized intersections compare with nonchannelized intersections.

(b) A channelized right-turn lane at a 90-degree intersection with an exclusive use (acceleration) lane on the receiving street. Under this geometric configuration, drivers did not need to stop at the intersection and they were removed from the conflicting traffic upon entering the cross street. They had the opportunity to accelerate in their own lane on the cross street and then change lanes downstream when they perceived that it was safe to do so.

(c) A channelized right-turn lane at a 65-degree skewed intersection without an exclusive use lane on the receiving street.

(d) A channelized right-turn lane at a 90-degree intersection without an exclusive use lane on the receiving street. Under this geometry, drivers needed to check the conflicting traffic and complete their turn into a through traffic lane on the cross street.

The right-turn maneuver at all locations was made against two lanes carrying through (conflicting) traffic. The two through lanes were the only ones that had a direct effect on the right-turn maneuver. All intersections were located on major or minor arterials within a growing urban area, where the posted speed limit was 56 km/h (35 mi/h). All intersections were controlled by traffic signals with yield control on the three channelized intersections.

The results indicated that right-turn channelization affects the speed at which drivers make right turns and the likelihood that they will stop before making an RTOR. Drivers, especially younger drivers (ages 25­45), turned right at speeds 4.8­8 km/h (3­5 mi/h) higher on intersection approaches with channelized right-turn lanes than they did on approaches with nonchannelized right-turn lanes.

At the nonchannelized intersection, 22 percent of the young/middle-aged drivers, 5 percent of the young-old drivers, and none of the old-old drivers performed an RTOR without a stop. On approaches with channelized right-turn lanes, young/middle-aged and young-old drivers were much less likely to stop before making an RTOR. Where an acceleration lane was available, 65 percent of the young/middle-aged drivers continued through without a complete stop, compared with 55 percent of the young-old drivers and 11 percent of the old-old drivers. Old-old female drivers always stopped before an RTOR. The increased mobility exhibited by the two younger groups of drivers at the channelized right-turn lane locations was not, however, exhibited by the old-old drivers (age 75 and older), who stopped in 19 of the 20 turns executed at the channelized locations. Also, questionnaire results indicated drivers perceived that making a right turn on an approach with a channelized right-turn lane without an acceleration lane on the cross street was more difficult than at other locations, and even more difficult than at skewed intersections.

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Table 4. Cross-references of related entries for intersection sight distance.

Because at-grade intersections define locations with the highest probability of conflict between vehicles, adequate sight distance is particularly important. Not surprisingly, a number of studies have shown that sight distance problems at intersections usually result in a higher accident rate (Mitchell, 1972; Hanna, Flynn, and Tyler, 1976; David and Norman, 1979). The need for adequate sight distance at an intersection is best illustrated by a quote from the Green Book: "The operator of a vehicle approaching an intersection at-grade should have an unobstructed view of the entire intersection and sufficient lengths of the intersecting highway to permit control of the vehicle to avoid collisions" (AASHTO, 1994). AASHTO values (for both uncontrolled and stop-controlled intersections) for available sight distance are measured from the driver's eye height (currently 1,070 mm [3.25 ft]) to the roofline of the conflicting vehicle (currently 1,300 mm [4.25 ft]).

Sight distances at an intersection can be reduced by a number of deficiencies, including physical obstructions too close to the intersection, severe grades, and poor horizontal alignment. The alignment and profile of an intersection have an impact on the sight distance available to the driver and thus affect the ability of the driver to perceive the actions taking place both at the intersection and on its approaches. Since proper perception is the first key to performing a safe maneuver at an intersection, it follows that sight distance should be maximized; this, in turn, means that the horizontal alignment should be straight and the gradients as flat as practical. Horizontal curvature on the approaches to an intersection makes it difficult for drivers to determine appropriate travel paths, because their visual focus is directed along lines tangential to these paths. Kihlberg and Tharp (1968) showed that accident rates increased 35 percent for highway segments with curved intersections over highway segments with straight intersections. Limits for vertical alignment at intersections suggested by AASHTO (1994) and Institute of Transportation Engineers (1984) are 3 and 2 percent, respectively.

Harwood, Mason, Pietrucha, Brydia, Hostetter, and Gittings (1993) stated that the provision of intersection sight distance (ISD) is intended to give drivers an opportunity to obtain the information they need to make decisions about whether to proceed, slow, or stop in situations where potentially conflicting vehicles may be present. They noted that while it is desirable to provide a reasonable margin of safety to accommodate incorrect or delayed driver decisions, there are substantial costs associated with providing sight distances at intersections; therefore, it is important that ISD requirements not be overly conservative or attempt to address traffic situations that are infrequent or unusual and for which increased ISD would provide little safety benefit.

Driver age differences in cognitive and physical capabilities that are relevant to ISD issues are discussed below. This is followed by a discussion of research efforts that have attempted to quantify the safety impact of providing adequate sight distance, plus studies that have examined the adequacy of the components that must be taken into account when calculating sight distance.

Older road users do not necessarily react more slowly to events that are expected, but they take significantly longer to make decisions about the appropriate response than younger road users, and this difference becomes more exaggerated in complex situations. Although the cognitive aspects of safe intersection negotiation depend upon a host of specific functional capabilities, the net result is response slowing. There is general consensus among investigators that older adults tend to process information more slowly than younger adults, and that this slowing not only transcends the slower reaction times often observed in older adults but may, in part, explain them (Anders, Fozard, and Lillyquist, 1972; Eriksen, Hamlin, and Daye, 1973; Waugh, Thomas, and Fozard, 1978; Salthouse and Somberg, 1982; Byrd, 1984). Of course, a conflict must be seen before any cognitive processing of this sort proceeds. Therefore, any decrease in available response time because of sight distance restrictions will pose disproportionate risks to older drivers. Slower reaction times for older versus younger adults when response uncertainty is increased has been demonstrated by Simon and Pouraghabagher (1978), indicating a disproportionately heightened degree of risk when older road users are faced with two or more choices of action. Also, research has shown that older persons have greater difficulty in situations where planned actions must be rapidly altered (Stelmach, Goggin, and Amrhein, 1988). The difficulty older persons experience in making extensive and repeated head movements further increases the decision and response times of older drivers at intersections.

David and Norman (1979) quantified the relationship between available sight distance and the expected reduction in accidents at intersections. The results of this study showed that intersections with shorter sight distances generally have higher accident rates. Using these results, predicted accident reduction frequencies related to ISD were derived as shown in table 5.

Other studies have attempted to show the benefits to be gained from improvements to ISD (Mitchell, 1972; Strate, 1980). Mitchell conducted a before-and-after analysis, with a period of 1 year on each end, of intersections where a variety of improvements were implemented. The results showed a 67 percent reduction (from 39 to 13) in accidents where obstructions that inhibited sight distance were removed; this was the most effective of the implemented improvements. Strate's analysis examined 34 types of improvements made in Federal Highway Safety Program projects. The results indicated that sight distance improvements were the most cost-effective, producing a benefit/cost ratio of 5.33:1.

Table 5. Expected reduction in number of accidents
per intersection per year. Source: David and Norman, 1979.

Collectively, the studies described above indicate a positive relationship between available ISD and a reduction in accidents, though the amount of accident reduction that can be expected by a given increase in sight distance may be expected to vary according to the maneuver scenario and existing traffic control at the intersection. Procedures for determining appropriate ISDs are provided by AASHTO for various levels of intersection control and the maneuvers to be performed. The scenarios defined are as follows:

• Case I: No Control. ISD for vehicles approaching intersections with no control, at which vehicles are not required to stop, but may be required to adjust speed.
  
• Case II: Yield Control. ISD for vehicles on a minor-road approach controlled by a yield sign.
  
• Case IIIA: Stop Control—Crossing Maneuver. ISD for a vehicle on a stop-controlled approach on the minor road to accelerate from a stopped position and cross the major road.
  
• Case IIIB: Stop Control—Left Turn. ISD for a vehicle on a stop-controlled approach on the minor road to accelerate from a stopped position and turn left onto the major road.
  
• Case IIIC: Stop Control—Right Turn. ISD for a vehicle on a stop-controlled approach on the minor road to accelerate from a stopped position and turn right onto the major road.
  
• Case IV: Signal Control (should be designed by Case III conditions). ISD for a vehicle on a signal-controlled approach.
  
• Case V: Stop Control—Vehicle Turning Left From Major Highway. ISD for a vehicle stopped on a minor road, waiting to turn left across opposing lanes of travel.

One of the principal components in determining ISD in all of these cases is perception-reaction time (PRT). The discussion of this value is first presented in chapters 2 and 3 of the Green Book under "Reaction Time" and "Brake Reaction Time," respectively (AASHTO, 1994). Results of several studies (e.g., Normann, 1953; Johansson and Rumar, 1971) are cited, and in conclusion, the 2.5-s value is selected since it was found to be adequate for approximately 90 percent of the overall driver population.

With respect to at-grade intersections, AASHTO recommends the following values of PRT for ISD calculations. In Case I, the PRT is assumed to be 2.0 s plus an additional 1.0 s to actuate braking, although the "preferred design" uses stopping sight distance (SSD) as the ISD design value (which incorporates a PRT of 2.5 s). In Case II, SSD is the design value; thus, the PRT is 2.5 s. For all Case III scenarios and Cases IV and V, the PRT is assumed to be 2.0 s.

A critique of these values questioned the basis for reducing the PRT from 2.5 s used in SSD calculations to 2.0 s in the Case III ISD calculations (Alexander, 1989). As noted by the author, "The elements of PRT are: detection, recognition, decision, and action initiation." For SSD, this is the time from object or hazard detection to initiation of the braking maneuver. Time to search for a hazard or object is not included in the SSD computation, and the corresponding PRT value is 2.5 s. Yet, in all Case III scenarios, the PRT has been reduced to 2.0 s and now includes a search component which was not included in the SSD computations. Alexander pointed out that a driver is looking straight ahead when deciding to perform a stopping maneuver and only has to consider what is in his/her forward view. At an intersection, however, the driver must look forward, to the right, and to the left. This obviously takes time, especially for those drivers with lower levels of physical dexterity, e.g., older drivers. Alexander (1989) proposed the addition of a "search time" variable to the current equations for determining ISD, and use of the PRT value currently employed in the SSD computations (i.e., 2.5 s) for all ISD computations. Neuman (1989) also argued that a PRT of 2.5 s for SSD may not be sufficient in all situations, and can vary from 1.5 s to 5.0 s depending on the physical state of the driver (alert versus fatigued), the complexity of the driving task, and the location and functional class of the highway.

A number of research efforts have been conducted to determine appropriate PRT values for use in ISD computations. Hostetter, McGee, Crowley, Seguin, and Dauber (1986) examined the PRT of 124 subjects traversing a 3-hour test circuit which contained scenarios identified above as Cases II, IIIA, IIIB, and IIIC. For the Case II (yield control) scenario, the results showed that in over 90 percent of the trials, subjects reacted in time to meet the SSD criteria established and thus the 2.5-s PRT value was adequate. With respect to Case III scenarios, the PRT was measured from the first head movement after a stop to the application of the accelerator to enter the intersection. The mean and 85th percentile values for all maneuvers combined were 1.82 s and 2.7 s, respectively. The results also showed that the through movement produced a lower value than the mean, while the turning maneuvers produced a higher value. These results lead to conclusions that the 2.0-s criteria for Case IIIA be retained and that the PRT value for the Case III turning maneuvers (B and C) be increased from 2.0 to 2.5 s. One other result, which is applicable to the current effort, was that no significant differences were found with respect to age, i.e., increased PRTs were needed to accommodate all drivers.

Another effort examined the appropriateness of the PRT values currently specified by AASHTO for computing SSD, vehicle clearance interval, sight distance on horizontal curves, and ISD (McGee and Hooper, 1983). With respect to ISD, the results showed the following: for Case I, the driver is not provided with sufficient time or distance to take evasive action if an opposing vehicle is encountered; and for Case II, adequate sight distance to stop before arriving at the intersection is not provided despite the intent of the standard to enable such action. With respect to the PRT values, recommendations include increasing the 2.0-s and 2.5-s values used in Case I and Case II calculations, respectively, to 3.4 s. It was also recommended that the PRT value for Case III scenarios be redefined.

Although there is no consensus from the above studies on the actual values of PRT that should be employed in the ISD computations, there is a very clear concern as to whether the current values are meeting the needs of older drivers. Since older drivers tend to take longer in making a decision, especially in complex situations, the need to further evaluate current PRT values is underscored. Slowed visual scanning of traffic on the intersecting roadway by older drivers has been cited as a cause of near misses of (crossing) accidents at intersections during on-road evaluations. In the practice of coming to a stop, followed by a look to the left, then to the right, and then back to the left again, the older driver's slowed scanning behavior allows approaching vehicles to have closed the gap by the time a crossing maneuver finally is initiated. The traffic situation has changed when the older driver actually begins the maneuver, and drivers on the main roadway are often forced to adjust their speed to avoid a collision. Hauer (1988) stated that "the standards and design procedures for intersection sight triangles should be modified because there is reason to believe that when a passenger car is taken as the design vehicle, the sight distance is too short for many older drivers, who take longer to make decisions, move their heads more slowly, and wish to wait for longer gaps in traffic."

In contrast, recent research conducted by Lerner, Huey, McGee, and Sullivan (1995) concluded that, based on older driver performance, no changes to design PRT values were recommended for ISD, SSD, or decision sight distance (DSD), even though the 85th percentile J values exceeded the AASHTO 2.0-s design standard at 7 of the 14 sites. The J value equals the sum of the PRT time and the time to set the vehicle in motion, in seconds. No change was recommended because the experimental design represented a worst-case scenario for visual search and detection (drivers were required to begin their search only after they had stopped at the intersection and looked inside the vehicle to perform a secondary task).

Lerner et al. (1995) conducted an on-road experiment to investigate whether the assumed values for Case III driver PRT used in AASHTO design equations adequately represent the range of actual PRT for older drivers. Approximately 33 subjects in each of three driver age groups were studied: ages 20­40, ages 65­69, and age 70 and older. Drivers operated their own vehicles on actual roadways, were not informed that their response times were being measured, and were naive as to the purpose of the study (i.e., they were advised that the purpose of the experiment was to judge road quality and how this relates to aspects of driving). The study included 14 data collection sites on a 90-km (56-mi) route. Results showed that the older drivers did not have longer PRT than younger drivers, and in fact the 85th percentile PRT closely matched the AASHTO design equation value of 2.0 s. The 90th percentile PRT was 2.3 s, with outlying values of 3­4 s. The median daytime PRT was approximately 1.3 s. Interestingly, it was found that typical driver actions did not follow the stop/search/decide maneuver sequence implied by the model; in fact, drivers continued to search and appeared ready to terminate or modify their maneuver even after they had begun to move into the intersection. This finding resulted in the study authors' conclusion that the behavioral model on which ISD is based is conservative.

Harwood, Mason, Brydia, Pietrucha, and Gittings (1996) evaluated current AASHTO policy on ISD for Cases I, II, III, IV, and V during performance of NCHRP project 15-14(1), based on a survey of current highway agencies' practices and a consideration of alternative ISD models and computational methodologies, as well as findings from observational studies for selected cases. Although this work culminated in recommendations for minimum distances for the major and minor legs of the sight triangle for all cases, driver age was not included as a study variable; therefore, specific values for these design elements were not included within the recommendations presented in this Handbook, nor is an exhaustive discussion of these materials included in this section. The results of the Harwood et al. (1996) analyses pertaining to ISD for Case IIIB and IIIC—and by extension for Case V—are of particular interest, however, in the interpretation of other, related findings from an older driver field study in this area. These analysis outcomes are reviewed below.

Prior to the 1990 AASHTO Green Book, the issue of ISD for a driver turning left off of a major roadway onto a minor roadway or into an entrance (Case V) was not specifically addressed. In the 1990 Green Book, the issue was addressed at the end of the Case III discussions in two paragraphs. In the 1994 Green Book, these same paragraphs have been placed under a new condition referred to as Case V. The equation used for determining ISD for Case V was simply taken from the Case IIIA (crossing maneuver at a stop-controlled intersection) and Case IIIB (left turn maneuver from a stop-controlled minor road onto a major road) conditions, with the primary difference between the cases being the distance traveled during the maneuver. A central issue in defining the ISD for Case V involves a determination of whether the tasks that define ISD for Cases IIIA and IIIB are similar enough to the tasks associated with Case V to justify using the same equation, which follows:

ISD = 1.47 V (J +t_a)English
ISD = 0.278 V (J +t_a)Metric


where:

ISD = intersection sight distance (feet for English equation; meters for metric equation).
V = major roadway operating speed (mi/h for English equation; km/h for metric equation).
J = time required to search for oncoming vehicles, to perceive that there is sufficient time to make the left turn, and to shift gears, if necessary, prior to starting (J is currently assumed to be 2.0 s).
t_a= time required to accelerate and traverse the distance to clear traffic in the approaching lane(s); obtained from figure IX-33 in the AASHTO Green Book.

For Case IIIA (crossing maneuver), the sight distance is calculated based on the need to clear traffic on the intersecting roadway on both the left and right sides of the crossing vehicle. For Case IIIB (left turn from a stop), sight distance is based on the requirement to first clear traffic approaching from the left and then enter the traffic stream of vehicles from the right. It has been demonstrated that the perceptual judgments required of drivers in both of these maneuver situations increase in difficulty when opposing through traffic must be considered.

The perceptual task of turning left from a major roadway at an unsignalized intersection or during a permitted signal phase at a signalized intersection requires a driver to make time-distance estimates of a longitudinally moving target as opposed to a laterally moving target. Lateral movement (also referred to as tangential movement) describes a vehicle that is crossing an observer's line of sight, moving against a changing visual background where it passes in front of one fixed reference point after another. Longitudinal movement, or movement in depth, results when the vehicle is either coming toward or going away from the observer. In this case there is no change in visual direction, only subtle changes in the angular size of the visual image, typically viewed against a constant background. Longitudinal movement is a greater problem for drivers because the same displacement of a vehicle has a smaller visual effect than when it moves laterally—that is, lateral movement results in a much higher degree of relative motion (Hills, 1980).

In comparison with younger subjects, a significant decline for older subjects has been reported in angular motion sensitivity. In a study evaluating the simulated change in the separation of taillights indicating the overtaking of a vehicle, Lee (1976) found a threshold elevation greater than 100 percent for drivers ages 70­75 compared with drivers ages 20­29 for brief exposures at night. Older persons may in fact require twice the rate of movement to perceive that an object's motion in depth is approaching, versus maintaining a constant separation or receding, given a brief duration (2.0 s) of exposure. In related experiments, Hills (1975) found that older drivers required significantly longer to perceive that a vehicle was moving closer at constant speed: at 31 km/h (19 mi/h), decision times increased 0.5 s between ages 20 and 75. This body of evidence suggests that the 2.0-s PRT (i.e., variable J in the ISD equation above) used for Cases III and V may not be sufficient for the task of judging gaps in opposing through traffic by older drivers. A revision of Case V to determine a minimum required sight distance value which more accurately reflects the perceptual requirements of the left-turn task may therefore be appropriate.

Harwood et al. (1996) suggested that at locations where left turns from the major road are permitted at intersections and driveways, at unsignalized intersections, and at signalized intersections without a protected turn phase, sight distance along the major road should be provided based on a critical gap approach, as was recommended for left and right turns from the minor road at stop-controlled intersections. The gap acceptance model developed and proposed to replace the current ISD AASHTO model is:

ISD = 1.47 VGEnglish
ISD = 0.278 VGMetric

where:

ISD =intersection sight distance (feet for English equation; meters for metric equation).
V =operating speed on the major road (mi/h for English equation, km/h for metric equation).
G =the specified critical gap (in seconds); equal to 5.5 s for crossing one opposing lane plus an additional 0.5 s for each additional opposing lane.

Field data were collected in the NCHRP study to better quantify the gap acceptance behavior of passenger car and truck drivers, but only for left- and right-turning maneuvers from minor roadways controlled by a STOP sign (Cases IIIB and C). In the Phase I interim report produced during the conduct of the NCHRP project, Harwood et al. (1993) reported that the critical gap currently used by the California Department of Transportation is 7.5 s. When current AASHTO Case IIIB ISD criteria are translated to time gaps in the major road traffic stream, the gaps range from 7.5 s (67 m [220 ft]) at a 32-km/h (20-mi/h) operating speed to 15.2 s (475 m [1,560 ft]) at a 112-km/h (70-mi/h) operating speed. Harwood et al. (1993) stated that the rationale for gap acceptance as an ISD criterion is that drivers safely accept gaps much shorter than 15.2 s routinely, even on higher speed roadways.

In developing the gap acceptance model for Case V, Harwood et al. (1996) relied on data from studies conducted by Kyte (1995) and Micsky (1993). Kyte (1995) recommended a critical gap value of 4.2 s for left turns from the major road by passenger cars for inclusion in the unsignalized intersection analysis procedures presented in the Highway Capacity Manual (Transportation Research Board, 1994). A constant value was recommended regardless of the number of lanes to be crossed; however, a heavy-vehicle adjustment of 1.0 s for two-lane highways and 2.0 s for multilane highways was recommended. Harwood et al. (1996) reported that Micsky's 1993 evaluation of gap acceptance behavior for left turns from the major roadway at two Pennsylvania intersections resulted in critical gaps with a 50 percent probability of acceptance (determined from logistic regression) of 4.6 s and 5.3 s. Using the rationale that design policies should be more conservative than operational criteria such as the Highway Capacity Manual, Harwood et al. (1996) recommended a critical gap for left turns from the major roadway of 5.5 s, and an increase in the critical gap to 6.5 s for left turns by single-unit trucks and to 7.5 s for left turns by combination trucks. In addition, if the number of opposing lanes to be crossed exceeds one, an additional 0.5 s per additional lane for passenger cars and 0.7 s per additional lane for trucks was recommended.

It is important to note that the NCHRP study did not consider driver age as a variable. However, Lerner et al. (1995) collected judgments about the acceptability of gapsin traffic as a function of driver age for left turn, right turn, and through movements at stop-controlled intersections. While noting that these authors found no significant differences between age groups in the total time required to perceive, react, and complete a maneuver in a related Case III PRT study, the Lerner et al. (1995) findings indicate that younger drivers accept shorter gaps than older drivers. The 50th percentile gap acceptance point was about 7 s (i.e., if a gap is 7 s long, only about half of the subjects would accept it). The 85th percentile point was approximately 11 s. The oldest group required about 1.1 s longer than the youngest group.

In an recently completed observational field study of driver performance as a function of left-turn lane geometry, mean left-turn critical gap sizes (in seconds) across four locations where the main road operating speed was 56 km/h (35 mi/h), for drivers who had positioned their vehicles within the intersection, were 5.90 s (young/middle-aged [ages 25­45] females), 5.91 s (young/middle-aged [ages 25­45] males), 6.01 s (young-old [ages 65­74] females), 5.84 s (young-old [ages 65­74] males), 6.71 s (old-old [age 75 and older] females), and 6.55 s (old-old [age 75 and older] males). Prominent trends indicated that older drivers demonstrated larger critical gap values at all locations. The young/middle-aged and young-old groups were not significantly different from each other; however, both were significantly different from the old-old group. Critical gap sizes displayed in a laboratory simulation study in the same project, where oncoming vehicles traveling at 56 km/h (35 mi/h) were viewed on a large screen display in correct perspective, ranged from 6.4 s to 8.1 s for young/middle-aged drivers and from 5.8 to 10.0 s for drivers age 75 and older (Staplin, Harkey, Lococo, and Tarawneh, 1997).

These diverse findings argue that an appropriate value for G in the gap acceptance model will lie toward the upper end of the 7- to 11-s range to accommodate older drivers, while also preserving a margin of safety. This strategy acknowledges the diminished capability of older drivers to judge oncoming vehicle speed in a situation that places this group of road users at particular risk, i.e., when an opposing through vehicle approaches at excessive speed.

Regarding PRT for Cases III and V, AASHTO (1994) assumes a PRT of 2.0 s as the time necessary for the driver to look in both directions of the roadway, to perceive that there is sufficient time to perform the maneuver safely, and to shift gears, if necessary, prior to starting. This value is based on research performed by Johansson and Rumar (1971). The PRT is defined as the time from the driver's first look for possible oncoming traffic to the instant the car begins to move. Some of these operations are done simultaneously by many drivers, and some operations, such as shifting gears, may be done before searching for intersecting traffic or may not be required with automatic transmissions. AASHTO states that a value of 2.0 s is assumed to represent the time taken by the slower driver.

Regarding the value of t_a, which is read from figure IX-33 in the AASHTO Green Book, the Staplin et al. (1997) data found no significant differences in maneuver time as a function of age for the drivers turning left at the four intersections studied (which had distances ranging from 26 to 32 m [84 to 106 ft]). Maneuver times for drivers positioned within the intersection versus unpositioned drivers, however, were significantly different. Since significantly fewer older drivers positioned themselves in the field study, the design value for this factor (maneuver time) should be based on that obtained for unpositioned drivers. A comparison of the 95th percentile clearance times demonstrated by positioned drivers and unpositioned drivers at each location with AASHTO values is presented in table 6.

Current and proposed sight distance models were exercised by Staplin et al. (1997) using data collected in the observational field study. For this comparison, two basic models were selected. Model 1 was the current model in the AASHTO Green Book for computing ISD, which relied on a PRT of 2.0 s and maneuver time taken from figure IX-33 in the Green Book. Model 2 was the gap acceptance model developed as part of NCHRP project 15-14(1), which relies on the critical gap in place of PRT and maneuver time. Model 2 takes the form shown below, with all terms as defined on page 56 in this section:

ISD = 1.47 VG English
ISD = 0.278 VGMetric

Each of these models was used with the appropriate design values to compute the required sight distance at each of the selected intersections. The models were then used with adjusted design values or actual data collected in the field to also determine the required sight distance at each location.

Table 6. Comparison of clearance times obtained in the Staplin et al. (1997) field study with AASHTO Green Book values used in sight distance calculations.

Measure	Vehicle Location	Left-Turn Lane Geometry
		-14-ft Offset	-3-ft Offset	0-ft Offset	+6-ft Offset
Distance Traveled (ft)	Positioned	70 ft	67 ft	64 ft	70 ft
95th Percentile Clearance Time (s) From Field Study	Positioned	3.8 s	3.9 s	3.9 s	3.9 s
AASHTO Clearance Time (s) From Figure IX-33	Positioned	5.1 s	5.0 s	5.0 s	5.1 s
Distance Traveled (ft)	Unpositioned	106 ft	98 ft	84 ft	88 ft
95th Percentile Clearance Time (s) From Field Study	Unpositioned	6.7 s	6.4 s	6.6 s	5.7 s
AASHTO Clearance Time (s) From Figure IX-33	Unpositioned	6.5 s	6.2 s	5.9 s	6.0 s

1 ft = 0.305 m

The first adjustment made to the current AASHTO model was an increase in the PRT. As previously noted, several studies have examined and critiqued the use of 2.0 s for PRT in this model. Thus, an adjusted model (Model 3) with a PRT of 2.5 s, which is equivalent to the value used in SSD calculations, was also included in the analysis.

One of the data elements collected as part of this research was the maneuver time of the left-turning driver. This time is equivalent to t_a in the AASHTO model. These times were measured from two locations, depending on whether or not the drivers positioned themselves within the intersection prior to turning. The first location was from a position within the intersection, approaching the median or centerline of the cross street. The second location was at or behind the stop bar or end of the left-turn bay. Using the original AASHTO model and these field data maneuver times, sight distances were computed with two additional models, substituting the 95th percentile maneuver time from the distribution of all unpositioned drivers in one model (Model 4) and the 95th percentile maneuver time from the distribution of all positioned drivers in the other model (Model 5).

Critical gap data were also collected and analyzed by driver age group, at each of the intersections studied. The drivers age 75 and older accepted significantly larger gaps compared with the other age groups. Thus, two different critical gaps were used in adjusted gap models to compute the required sight distances. These models simply modify the value of G in Model 2. In the first adjustment, the critical gap for all drivers (across age) as measured in the field was substituted for the value of G (Model 6); in the second adjustment, the critical gap for drivers age 75 and older as measured in the field was substituted for the value of G (Model 7).

A detailed discussion of the outputs from the model exercise is provided in the publication Intersection Geometric Design and Operational Guidelines for Older Drivers and Pedestrians (Staplin, Harkey, Lococo, and Tarawneh, 1997). However, one significant result is that the required sight distances computed using the modified AASHTO model (where PRT was increased to 2.5 s) produced required sight distance values that were the most predictive of actual field operations. Thus, if the current AASHTO model is deemed to be the appropriate one for calculating ISD as it relates to drivers turning left from a major roadway, there is evidence that the PRT value should be increased to 2.5 s to provide adequate sight distance at most locations. The gap acceptance model, on the other hand, produced sight distance values that were approximately 23 percent shorter than the current AASHTO model. If the gap acceptance model is going to be used, particularly where there are significant volumes of older left-turning drivers, there may need to be an adjustment factor applied to increase the sight distance to better accommodate this driver age group. Also, to the extent that the current AASHTO ISD model produces longer sight distances than the gap acceptance model, it may be most prudent—considering the increasing range of driver (diminished) capabilities—to regard the difference as simply an additional margin of safety.

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E. Design Element: Opposite (Single) Left-Turn Lane Geometry, Signing, and Delineation

Table 7. Cross-references of related entries for opposite (single) left-turn lane geometry, signing, and delineation.

The alignment of opposite left-turn lanes and the horizontal and vertical curvature on the approaches are the principal geometric design elements that determine how much sight distance is available to a left-turning driver. Operationally, vehicles in the opposite left-turn lane waiting to turn left can also restrict the (left-turning) driver's view of oncoming traffic in the through lanes. The level of blockage depends on how the opposite left-turn lanes are aligned with respect to each other, as well as the type/size of vehicles in the opposing queue. Restricted sight distance can be minimized or eliminated by offsetting opposite left-turn lanes so that left-turning drivers do not block each other's view of oncoming through traffic. When the two left-turn lanes are exactly aligned, the offset distance has a value of zero. Negative offset describes the situation where the opposite left-turn lane is shifted to the left. Positive offset describes the situation where the opposite left-turn lane is shifted to the right. Positively offset left-turn lanes and aligned left-turn lanes provide greater sight distances than negatively offset left-turn lanes, and a positive offset provides greater sight distance than the aligned configuration.

Older drivers may experience greater difficulties at intersections as the result of diminished visual capabilities such as depth and motion perception, as well as diminished attention-sharing (cognitive) capabilities. Studies have shown that there are age differences in depth and motion perception. Staplin, Lococo, and Sim (1993) found that the angle of stereopsis (seconds of arc) required for a group of drivers age 75 and older to discriminate depth using a commercial vision tester was roughly twice as large as that needed for an 18­55-year-old group to achieve the same level of performance. However, while accurate perception of the distance to geometric features delineated at intersections—as well as to potentially hazardous objects such as islands, pedestals, and other raised features—is important for the safe use of these facilities, relatively greater attention by researchers has been placed upon motion perception, where dynamic stimuli (usually other vehicles) are the primary targets of interest. It has been shown that older persons require up to twice the rate of movement to perceive that an object is approaching, and they require significantly longer to perceive that a vehicle is moving closer at a constant speed (Hills, 1975). A study investigating causes of older driver overinvolvement in turning accident at intersections, building on the previously reported decline for detection of angular expansion cues, did not find evidence of overestimation of time-to-collision (Staplin et al., 1993). At the same time, a relative insensitivity to approaching (conflict) vehicle speed was shown for older versus younger drivers; this result was interpreted as supporting the notion that older drivers rely primarily or exclusively on perceived distance—not time or velocity—to perform gap acceptance judgments, reflecting a reduced ability to integrate time and distance information with increasing age. Thus, a principal source of risk at intersections is the error of an older, turning driver in judging gaps in front of fast vehicles.

Several recent studies examining the minimum required sight distance for a driver turning left from a major roadway to a minor roadway, as a function of major road design speed, have provided data necessary to determine: (1) the left-turn lane offset value needed to achieve the minimum required sight distance and (2) the offset value that will provide unlimited sight distance. A fundamental premise in these studies, which are described below, is that it is not the amount of left-turn lane offset per se, but rather the sight distance which a given level of offset provides that should be the focus of any recommendations pertaining to the design of opposite left-turn lanes.

In a study conducted by McCoy, Navarro, and Witt (1992), guidelines were developed for offsetting opposite left-turn lanes to eliminate the left-turn sight distance problem. All minimum offsets specified in the guidelines are positive. For 90-degree intersections on level tangent sections of four-lane divided roadways, with 3.6-m (12-ft) left-turn lanes in 4.9-m (16-ft) medians with 1.2-m (4-ft) medial separators, the following conclusions were stated by McCoy et al.: (1) a 0.6-m (2-ft) positive offset provides unrestricted sight distance when the opposite left-turn vehicle is a passenger car, and (2) a 1.06-m (3.5-ft) positive offset provides unrestricted sight distance when the opposite left-turn vehicle is a truck, for design speeds up to 113 km/h (70 mi/h).

Harwood, Pietrucha, Wooldridge, Brydia, and Fitzpatrick (1995) conducted an observational field study and an accident analysis to develop design policy recommendations for the selection of median width at rural and suburban divided highway intersections based on operational and safety considerations. They found that at rural unsignalized intersections, both accidents and undesirable driving behaviors decrease as median width increases. However, at suburban signalized and unsignalized intersections, accidents and undesirable behaviors increase as the median width increases. At suburban intersections, it is therefore suggested that the median should not generally be wider than necessary to accommodate pedestrians and the appropriate median left-turn treatment needed to serve current and anticipated future traffic volumes. Harwood et al. stated that wider medians generally have positive effects on traffic operations and safety; however, wider medians can result in sight restrictions for left-turning vehicles due to the presence of opposite left-turn vehicles. The most common solution to this problem is to offset the left-turn lanes, using either parallel offset or tapered offset left-turn lanes.

Figure 3 compares conventional left-turn lanes with these two alternative designs. As noted by Harwood et al. (1995), parallel and tapered offset left-turn lanes are still not common, but are used increasingly to reduce the risk of accidents due to sight restrictions from opposite left-turn vehicles. Parallel offset left-turn lanes with 3.6-m (12-ft) widths can be constructed in raised medians with widths as narrow as 7.2 m (24 ft), and can be provided in narrower medians if restricted lane widths or curb offsets are used or a flush median is provided (Bonneson, McCoy, and Truby, 1993). Tapered offset left-turn lanes generally require raised medians of 7.2 m (24 ft) or more in width.

Staplin, Harkey, Lococo, and Tarawneh (1997) performed a laboratory study, field study, and sight distance analysis to measure driver age differences in performance under varying traffic and operating conditions, as a function of varying degrees of offset of opposite left-turn lanes at suburban arterial intersections. Research findings indicated that an increase in sight distance through positively offsetting left-turn lanes can be beneficial to left-turning drivers, particularly older drivers. In the field study, where left-turn vehicles needed to cross the paths of two or three lanes of conflicting traffic (excluding parking lanes) at 90-degree, four-legged intersections, four levels of offset of opposite left-turn lane geometry were examined. These levels include: (a) 1.8-m (6-ft) "partial positive" offset, (b) aligned (no offset) left-turn lanes, (c) 0.91-m (3-ft) "partial negative" offset, and (d) 4.3-m (14-ft) "full negative" offset. All intersections were located within a growing urban area where the posted speed limit was 56 km/h (35 mi/h). Additionally, all intersections were controlled by traffic-responsive semi-actuated signals, and all left-turn maneuvers were completed during the permitted left-turn phase at all study sites. In the analysis of the field study lateral positioning data, it was found that the partial positive offset and aligned locations had the same effect on the lateral positioning behavior of drivers. Drivers moved approximately 1.5 m (5 ft) to the left when there was a large negative offset, clearly indicating that sight distance was limited. There was a significant difference between the partial negative offset geometry and the partial positive offset or aligned geometries, suggesting a need for longer sight distance when opposite left-turn lanes are even partially negatively offset. The fact that older drivers (and females) were less likely to position themselves (i.e., pull into the intersection) in the field studies highlights the importance of providing adequate sight distance for unpositioned drivers, for all left-turn designs.

Several issues were raised in the research conducted by Staplin et al. (1997) regarding the adequacy of the current and proposed intersection sight distance (ISD) models for a driver turning left from a major roadway. The researchers exercised seven sight distance models, including the current AASHTO Case V model using 2.0 s for perception-reaction time (PRT), a modified AASHTO model using a 2.5-s PRT, and a gap acceptance model proposed by Harwood, Mason, Brydia, Pietrucha, and Gittings (1996). The NCHRP-proposed gap acceptance model relies on a critical gap value in place of PRT and maneuver time. A detailed description of the model parameters and output can be found in the FHWA report entitled Intersection Geometric Design and Operational Guidelines for Older Drivers and Pedestrians (Staplin et al., 1997). Of particular significance was the finding that the modified AASHTO model with the longer PRT of 2.5 s was the model most predictive of actual field operations. Also of significance was the dramatic decrease in required sight distance that occurred with the gap acceptance model compared with the traditional AASHTO model. Across all intersections and all design speeds, the required sight distance was approximately 23 percent less using the gap acceptance model. However, this was expected since the rationale behind the use of a gap acceptance model (cf. Harwood et al., 1996), in place of the current AASHTO model, is the fact that drivers are commonly observed accepting shorter gaps than those implied by the current model.

Regardless of which model is deemed most appropriate for computing ISD for drivers turning left off a major roadway, one way to increase the sight distance is through positively offset left-turn lanes. As shown in the study by Staplin et al. (1997), such designs result in significantly better performance on the part of all drivers, but especially for older drivers. Prior work by McCoy et al. (1992) examined the issue of offset left-turn lanes and developed an approach that could be used to compute the amount of offset that is required to minimize or eliminate the sight restriction caused by opposing left-turn vehicles. This approach was applied to the intersections in the study by Staplin et al. (1997) to determine the amount of offset that would be required when using the modified AASHTO model (i.e., J = 2.5 s). The left-turn lane offsets required to achieve the minimum required sight distances calculated using this model are shown in figure 4, in addition to the offsets required to provide unrestricted sight distance. Based on intersections examined in the study, the offset necessary to achieve unrestricted sight distance for opposing left-turning cars is 1.2 m (4.1 ft) and for opposing left-turning trucks is 1.7 m (5.6 ft).

Figure 4. Left-turn lane offset design values required to achieve minimum required
sight distances using the modified AASHTO model (J = 2.5 s) and unrestricted sight distances.

Finally, the potential for wrong-way maneuvers, particularly by older drivers, at intersections with positively offset channelized left-turn lanes was raised during a panel meeting comprised of older driver experts and highway design engineers, during the conduct of the research performed by Staplin et al. (1997). The concern expressed was that drivers turning left from the minor road may turn too soon and enter the channelized left-turn lane, instead of turning around both medians. Similar concern was raised by highway engineers surveyed by Harwood et al. (1995) during the conduct of NCHRP project 15-14(2). These authors reported that the potential for wrong-way movements by opposing-direction vehicles entering the left-turn roadway is minimal if proper signing and pavement markings are used. Researchers studying wrong-way movements at intersections—particularly the intersection of freeway exits with secondary roads—have found that such movements resulted from left-turning vehicles making an early left turn rather than turning around the nose of the median, and have proposed and tested several countermeasures. Parsonson and Marks (1979) found that the use of a wide (610 mm [24 in]) white stop bar was effective in preventing wrong-way entries onto freeway exit ramps in Georgia, as was the use of the two-piece, 7-m (23.5-ft) painted arrow pavement marking (wrong-way arrow). Scifres and Loutzenheiser (1975) reported that indistinct medians are design elements that reduce a driver's ability to see and understand the overall physical and operational features of an intersection, increasing the frequency of wrong-way movements. They suggested delineation of the median noses to increase their visibility and improve driver understanding of the intersection design and function.

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F. Design Element: Edge Treatments/Delineation of Curbs, Medians, and Obstacles

Table 8. Cross-references of related entries for edge treatments/delineation of curbs, medians, and obstacles.

The discrimination at a distance of gross highway features, as opposed to the fine detail contained in a sign message, governs drivers' perceptions of intersection geometric elements. Thus, the conspicuity of such elements as curbs, medians, and obstacles, as well as all raised channelization, is of paramount importance in the task of safely approaching and choosing the correct lane for negotiating an intersection, as well as avoiding collisions with the raised surfaces. During the conduct of their driving task analysis, McKnight and Adams (1970a, 1970b) identified five driving tasks related specifically to the conspicuity of intersection geometric elements: (1) maintain correct lateral lane position, (2) survey pavement markings, (3) survey physical boundaries, (4) determine proper lane position for the intended downstream maneuver, and (5) search for path guidance cues. The visual/perceptual requirement common to the performance of these tasks is contrast sensitivity: for detecting lane lines, painted roadway symbols and characters, curbs and roadway edge features, and median barriers.

Older drivers' decreased contrast sensitivity, reduced useful field of view, increased decision time—particularly in response to unexpected events—and slower vehicle control movement execution combine to put these highway users at greater accident risk when approaching and negotiating intersections. The smaller the attentional demand required of a driver to maintain the correct lane position for an intended maneuver, the greater the attentional resources available for activities such as the recognition and processing of traffic control device messages and detection of conflict vehicles and pedestrians.

A variety of conspicuity-enhancing treatments are mandated in current practice. The MUTCD (section 3B-13) specifies that pavement markings shall be used to guide traffic on the approach to fixed objects (such as channelization islands) within a paved roadway. Section 3B-21 (Curb Markings for Parking Restrictions) states that curb markings of yellow and white are used for delineation and visibility; section 3D-3 (Curb Markings for Delineation) states that reflectorized solid yellow (delineators) should be placed on the curbs of islands located in the line of traffic flow where the curb serves to channel traffic to the right of the obstruction, and reflectorized solid white (delineators) should be used when traffic may pass on either side of the island. Supplementary treatments, and requirements for in-service brightness levels for certain elements contained in existing guidelines, are presently at issue.

The conspicuity of curbs and medians, besides aiding in the visual determination of how an intersection is laid out, is especially important when medians are used as pedestrian refuges. Care must be taken to ensure that pedestrian refuges are clearly signed and made as visible as possible to passing motorists.

Research findings describing driver performance differences directly affecting the use of pavement markings and delineation focus upon (age-related) deficits in spatial vision. In a pertinent laboratory study conducted by Staplin, Lococo, and Sim (1990), two groups of subjects (ages 19­49 and 65­80) viewing a series of ascending and descending brightness delineation targets were asked to report when they could just detect a roadway heading—left versus right—from simulated distances of 30.5 m (100 ft) and 61 m (200 ft). Results showed that the older driver group required a contrast of 20 percent higher than the younger driver group to achieve the discrimination task in this study.

To describe the magnitude of the effects of age and visual ability on delineation detection/recognition distance and retroreflective requirements for threshold detection of pavement markings, a series of analyses using the Ford Motor Company PC DETECT computer model (cf. Matle and Bhise, 1984) yielded the stripe contrast requirements shown in table 9. PC DETECT is a headlamp seeing-distance model which uses the Blackwell and Blackwell (1971, 1980) human contrast sensitivity formulations to calculate the distance at which various types of targets illuminated by headlamps first become visible to approaching drivers, with and without glare from opposing headlights. The top 5 percent of 25-year-olds (the best-performing younger drivers) and bottom 5 percent of 75-year-olds (the worst-performing older drivers) were compared in this analysis.

Blackwell and Taylor (1969) conducted a study of surface pavement markings employing an interactive driving simulator, plus field evaluations. They concluded that driver performance —measured by the probability of exceeding lane limits—was optimized when the perceived brightness contrast between pavement markings and the roadway was 2.0. A study by Allen, O'Hanlon, and McRuer (1977) also concluded that delineation contrast should be maintained above a value of 2.0 for adequate steering performance under clear night driving conditions. In other words, because contrast is defined as the difference between target and background Table 9. Contrast requirements for edgeline visibility at 122 m (400 ft) with 5-s preview at a speed of 88 km/h (55 mi/h), as determined by PC DETECT computer model.

luminance, divided by the background luminance alone, these studies have asserted that markings must appear to be at least three times as bright as the road surface. Also, because these studies were not specifically focused on the accommodation of older drivers—particularly the least capable members of this group—the contrast requirements defined in more recent modeling studies and analyses, as presented in table 9, are accorded greater emphasis. Taking the indicated value for the least capable 5 percent of 75-year-olds into account, as well as the prior field evaluations, a contrast requirement of 3.0 for pavement markings appears most reasonable.

Note that luminance may be measured in candelas per square meter (cd/m²) or in footlamberts (fL), but contrast is a dimensionless number; thus the calculation of contrast level is independent of the unit of measure.

Finally, inadequate conspicuity of raised geometric features at intersections has been brought to the attention of researchers during the conduct of several focus group studies involving older drivers. Subjects reported difficulty knowing where to drive, due to missing or faded roadway lines on roadway edges and delineation of islands and turning lanes. They also reported hesitating during turns, because they did not know where to aim the vehicle (Staplin, Lococo, and Sim, 1990). In another focus group, subjects suggested that the placement of advance warning pavement markings be located as far in advance of an intersection as practicable (Council and Zegeer, 1992). Drivers ages 66­77 and older participating in focus group discussions conducted by Benekohal, Resende, Shim, Michaels, and Weeks (1992), reported that intersections with too many islands are confusing because it is hard to find which island the driver is supposed to go around. Raised curbs that are unpainted are difficult to see, especially in terms of height and direction, and result in people running over them. These older drivers stated that they would prefer to have rumble strips in the pavement to warn them of upcoming concrete medians and to warn them about getting too close to the shoulder. In more recent focus group discussions conducted to identify intersection geometric design features that pose difficulty for older drivers and pedestrians (Staplin, Harkey, Lococo, and Tarawneh, 1997), drivers mentioned that they have problems seeing concrete barriers in the rain and at night, and characterized barriers as "an obstruction waiting to be hit."

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G. Design Element: Curb Radius

Table 10. Cross-references of related entries for curb radius.

Curb radii, simply defined, are the radii of curves that join the curbs of adjacent approaches. The size of the radii affects the size of vehicle that can turn at the intersection, the speed at which vehicles can turn, and the width of intersection that must be crossed by pedestrians. If the curb radii are too small, lane encroachments resulting in traffic conflicts and increased accident potential can occur. If the radii are too large, pedestrian exposure may be increased (although, if large enough, refuge islands may be provided). The procedures used in the design of curb radii are well detailed in the Green Book (AASHTO, 1994).

McKnight and Stewart (1990) identified the task of positioning a vehicle in preparation for turning as a critical competency. A significant problem identified in a task analysis to prioritize older drivers' problems with intersections is carrying out the tight, right-turn maneuver at normal travel speed on a green light (Staplin, Lococo, McKnight, McKnight, and Odenheimer, 1994). The problems are somewhat moderated when right turns are initiated from a stop, because the turn can be made more slowly, which reduces difficulties with short radii. Older drivers may seek to increase the turning radius by moving to the left before initiating the turn, often miscommunicating an intent to turn left and encouraging following drivers to pass on the right. Or, they may initiate the turn from the correct position, but swing wide into a far lane in completing the turn in order to lengthen the turning radius and thus minimize rotation of the steering wheel. Encroaching upon a far lane can lead to conflict with vehicles approaching from the right or, on multilane roads, oncoming drivers turning to their left at the same time. The third possibility is to cut across the apex of the turn, possibly dragging the rear wheels over the curb. Each of these shortcomings in lanekeeping can be overcome by a channelized right-turn lane or wider curb radii.

Chu (1994) found that relative to middle-aged drivers (ages 25­64), older drivers (age 65 and older) tend to drive larger automobiles and drive at slower speeds. Although large heavy cars are associated with an accident fatality rate that is less than one-quarter of that associated with the smallest passenger cars (Insurance Institute for Highway Safety, 1991) and are, therefore, a wise choice for older drivers who are more frail than their middle-aged counterparts, large vehicles have a larger turning radius, which may exacerbate the problems older drivers exhibit in lanekeeping during a turn. Roberts and Roberts (1993) reported that common arthritic illnesses such as osteoarthritis, which affects more than 50 percent of the elderly population, and rheumatoid arthritis, affecting 1 to 2 percent, are relevant to the tasks of turning and gripping the steering wheel. A hand deformity caused by either osteoarthritis or rheumatoid arthritis may be very sensitive to pressure, making the driver unwilling to apply full strength to the steering wheel or other controls. In an assessment of 83 drivers with arthritis, Cornwell (1987) found that 83 percent of the arthritic group used both hands to steer, 7 percent used the right hand only, and 10 percent the left hand only; in this study, more than one-half of the arthritic group required steering modifications, either in the form of power steering or other assistive device such as a smaller steering wheel.

The Intersection Channelization Design Guide (Neuman, 1985) states that intersections on high-speed roadways with smooth alignment should be designed with sufficient radii to accommodate moderate- to high-speed turns. At other intersections, such as in residential neighborhoods, low-speed turns are desirable, and smaller corner radii are appropriate in these cases. Additionally, selection of a design vehicle is generally based on the largest standard or typical vehicle type that would regularly use the intersection. For example, a corner radius of 15 m (50 ft) will accommodate moderate-speed turns for all vehicles up to WB-50 (combination truck/large semitrailer with an overall length of 17 m [55 ft]). However, many agencies are designing intersections along their primary systems to accommodate a 21-m (70-ft), single trailer design vehicle (C-70). Table 4-8 (p. 66) of the Intersection Channelization Design Guide provides guidelines for the selection of a design vehicle. It further specifies in table 4-9 what the operational characteristics are of various corner radii. For example, a corner radius of less than 1.5 m (5 ft) is not appropriate even for P design vehicles (passenger cars), whereas a corner radius of 6­9 m (20­30 ft) will accommodate a low-speed turn for P vehicles, and a crawl-speed turn for SU vehicles (single unit truck, 9 m [30 ft] in length) with minor lane encroachment.

Of equal importance to the consideration of the right-turning design vehicle in determining curb radii is a consideration of pedestrian crossing time, particularly in urban areas. Smaller corner radii (less than 9 m [30 ft]) can decrease right-turn speeds and reduce open pavement area for pedestrians crossing the street. A consideration of vehicle turning speed and pedestrian crossing distance can contribute to the safe handling of vehicle/pedestrian crossing conflicts (Neuman, 1985). Hauer (1988) noted that "the larger the curb-curve radius, the larger the distance the pedestrian has to cover when crossing the road. Thus, for a sidewalk whose centerline is 1.8 m (6 ft) from the roadway edge, a 4.5-m (15-ft) corner radius increases the crossing distance by only 1 m (3 ft). However, a 15-m (50-ft) radius increases this distance by 8 m (26 ft), or 7 s of additional walking time." Hauer further stated that the following are widely held concerns with the widening of curb radii: (1) the longer the crossing distance, the greater the hazard to pedestrians, even though there may be space for refuge islands when the corner radius is large enough; (2) larger curb radii may induce drivers to negotiate the right turn at a higher speed; and (3) the larger the radius, the wider the turn, which makes it more difficult for the driver and the pedestrian to see each other. For these reasons, the safety of older persons at intersections, particularly pedestrians, may be adversely affected when large curb radii are provided.

In focus group discussions with 46 drivers ages 65­74 (young-old group) and 35 drivers age 75 and older (old-old group), 74 percent of drivers in each age group reported that tight intersection corner radii posed difficulty in maneuvering through the intersection for the following reasons: (1) there are visibility problem with sharp corners; (2) drivers sometimes hit curbs and median barriers; and (3) with sharp turns, trucks turning left into the adjacent opposing traffic lane end up face-to-face with drivers, requiring them to back up (Staplin, Harkey, Lococo, and Tarawneh, 1997). Approximately 24 percent of the young-old drivers and 34 percent of the old old drivers suggested that medium rounding is sufficient to facilitate turning maneuvers and is safer than very broadly rounded corners because the latter encourages high-speed turns.

In a design preference study using slides to depict varying radii of corner curb cuts, four alternative curb geometries were presented to 30 drivers ages 65­74 (young-old group) and 30 drivers age 75 and older (old-old group) (Staplin et al., 1997). The four alternative geometries (depicted in figure 5) were: (1) a simple circular radius of 5.5 m (18 ft); (2) a simple circular radius of 12 m (40 ft); (3) a simple circular radius of 14.6 m (48 ft); and (4) a three-sided/truncated curve with the center side measuring 16.5 m (54 ft).

Figure 5. Alternative curb radii evaluated in laboratory preference study conducted by Staplin et al. (1997).

The alternatives were identically ranked by both older samples: Alternative 3 was consistently preferred, Alternative 4 placed second, Alternative 2 placed third, and Alternative 1 was least preferred. Both young-old and old-old drivers in this study were most concerned about ease of turning, citing the better maneuverability and less chance of hitting the curb as their primary basis of response. The second most common—but also strongly weighted—reason for the preference responses of both groups related to the degree of visibility of traffic on intersecting roadways, possibly explaining the slight preference for Alternative 2 over Alternative 1. Alternatives 3 and 4 both are described by corner curbline geometries offering ease of turning and good visibility; however, isolated responses to the truncated corner geometry (Alternative 4) indicated concerns that too much room in the right-turn path might result in a lack of needed guidance information and could lead to a maneuver error, and that it could be harder to detect pedestrians with this design.

In a field study conducted as part of the same project, three intersections providing right turn curb radii of 12.2 m, 7.6 m, and 4.6 m (40 ft, 25 ft, and 15 ft) were evaluated to examine the effects of curb radii on the turning paths of vehicles driven by drivers in three age groups. One hundred subjects divided across three age groups drove their own vehicles around test routes using the local street network in Arlington, VA. The three age groups were "young/middle-aged" (ages 25-45), which contained 32 drivers; "young-old" (ages 65-74), containing 36 drivers; and "old old" (age 75 and older), containing 32 drivers. The speed limit was 56 km/h (35 mi/h) and all intersections were located on major or minor arterials within a growing urban area. Data were only collected for turns executed on a green-signal phase.

Analysis of the free-flow speeds showed that all factors (age, gender, and geometry), and their interactions, were significant. Mean free-flow speeds were highest at the largest (12.2-m [40 ft]) curb radius location, for all age groups. A consistent finding showed that the slowest mean free-flow speeds were measured at the 4.6-m (15-ft) curb radius location for all age groups. Thus, larger curb radii increased the turning speeds of all drivers, with young/middle-aged and young-old drivers traveling faster than old-old drivers when making right turns.

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Table 11. Cross-references of related entries for left-turn movements at signalized intersections.

Accident analyses have shown that older drivers, ages 56­75 and age 76 and older, are overinvolved in left-turn maneuvers at signalized intersections, with failure to yield right-of-way or disregarding the signal the principal violation types (Staplin and Lyles, 1991; Council and Zegeer, 1992). Old-elderly drivers (age 75 and older) were more likely than younger drivers (ages 30­50) to be involved in left-turn accidents at urban signalized intersections, and both young-elderly (ages 65­74) and old-elderly were more likely to be involved in left-turn accidents at rural signalized intersections. In both cases, the accident-involved older drivers were more likely to be performing a left-turn maneuver than the younger drivers. In addition, Stamatiadis, Taylor, and McKelvey (1991) found that the relative accident involvement ratios for older drivers were higher at two phase (no turning phase) signalized intersections than for multiphase (includes turn arrow) signalized intersections. This highlights problems older drivers may have determining acceptable gaps and maneuvering through traffic streams when there is no protective phase. Further, accident percentages increased significantly for older drivers when an intersection contained flashing controls, as opposed to conventional (red, yellow, green) operations. In this analysis, the greatest accident frequency at signalized intersections occurred on major streets with five lanes, followed closely by roadways containing four lanes. These configurations were most often associated with low-speed, high-volume urban locations, where intersection negotiation requires more complex decisions involving more conflict vehicles and more visually distracting conditions. Not surprisingly, Garber and Srinivasan's (1991) analysis of 7,000 intersection accidents involving drivers ages 50­64 and age 65 and older, found that the provision of a protected left-turn phase will aid in reducing the accident rates of the elderly at signalized intersections.

The change in the angular size of a moving object, such as an approaching vehicle observed by a driver about to turn left at an intersection, provides information crucial to gap judgments (i.e., speed and distance). Age-related declines (possibly exponential) in the ability to detect angular movement have been reported. Older persons may in fact require twice the rate of movement than younger persons to perceive that an object is approaching, given a brief (2.0 s) duration of exposure. Also, older persons participating in laboratory studies have been observed to require significantly longer than younger persons to perceive that a vehicle was moving closer at constant speed: at 31 km/h (19 mi/h), decision times increased 0.5 s between ages 20 and 75 (Hills, 1975).

Compounding this age-related decline in motion perception, some research has indicated that, relative to younger subjects, older subjects underestimate approaching vehicle speeds (Hills and Johnson, 1980). Specifically, Scialfa, Guzy, Leibowitz, Garvey, and Tyrrell (1991) showed that older adults tend to overestimate approaching vehicle velocities at lower speeds and underestimate at higher speeds, relative to younger adults. Staplin, Lococo, and Sim (1993), while investigating causes of older driver overinvolvement in turning accidents at intersections, did not find evidence of overestimation of time-to-collision by older drivers in their perception of the closing distance between themselves and another vehicle approaching either head-on or on an intersecting path. However, a relative insensitivity to approach (conflict) vehicle speed was shown for older versus younger drivers, in that younger drivers adjusted their gap judgment of the "last safe moment" to proceed with a turn appropriately to take higher approach speeds into account, while older drivers as a group failed to allow a larger gap for a vehicle approaching at 96 km/h (60 mi/h) than for one approaching at 48 km/h (30 mi/h). The interpretation of this and other data in this study was that older drivers rely primarily or exclusively on perceived vehicle separation distance to reach maneuver "go/no go" decisions, reflecting a reduced ability to integrate time and distance information with increasing age. Thus, a principal source of risk at intersections is the error of an older, turning driver in judging gaps in front of fast vehicles.

Aside from (conflict vehicle) motion detection, an additional concern is whether there are age differences in how well drivers understand the rules under which the turns will be made—that is, whether older drivers have disproportionately greater difficulty in understanding the message that is being conveyed by the signal and any ancillary (advisory) signs. If the signals and markings are not understood, at a minimum there may be delay in making a turn or, in the worst case, an accident could result if a protected operation is assumed where it does not exist.

A driver comprehension analysis conducted in a laboratory setting with drivers 30­60 years of age and older showed that green displays (those with the green ball alone, green arrow alone, or combinations of green ball and green arrow on the left-turn signal) were correctly interpreted with widely varying frequency, depending on the signals shown for the turning and through movements (Curtis, Opiela, and Guell, 1988). In most cases, performance declined as age increased; older drivers were correct approximately half as often as the youngest drivers. Most driver errors, and especially older driver errors, indicated signal display interpretations that would result in conservative behavior, such as stopping and/or waiting. A summary of the results follows. The simple green ball under permitted control was correctly interpreted by approximately 60 percent of the subjects. For protected-only operations, the green arrow (with red ball for through movement) was correctly answered by approximately 75 percent of drivers. For protected/permitted operation, the green ball alone was correctly answered by only 50 percent of the respondents, while the green arrow in combination with the green ball had approximately 70 percent correct responses. When the green ball with the green arrow was supplemented by the R10-12 sign LEFT TURN YIELD ON GREEN, only 34 percent of drivers answered correctly. This test result suggests that the MUTCD recommended practice may result in some driver confusion, as test subjects answered correctly more often when the sign was not present, even when the effects of regional differences in familiarity with the sign were considered. Green arrows were better understood than green balls. Conversely, red and yellow arrows were less comprehensible than red and yellow balls. Potentially unsafe interpretations were found for red arrow displays in protected-only operations. The yellow arrow display was more often treated as a last chance to complete a turn when compared with a yellow ball. Driver errors were most frequent in displays that involved flashing operations, and multiple faces with different colors illuminated on the left-turn signal head, and in particular, different colors on the turn and through signals.

When Hummer, Montgomery, and Sinha (1990) evaluated motorists' understanding of left turn signal alternatives, they found that the protected-only signal was by far the best understood, permitted signals were less understood, and the protected/permitted the least understood. When a green ball for through traffic and a green arrow for left turns were displayed, the protected signal was clearly preferred over the permitted and protected/permitted signals, and the leading signal sequence was preferred more often than the lagging sequence. Respondents stated that the protected-only signal caused less confusion, was safer, and caused less delay than the permitted and protected/permitted signals. It should be noted, however, that while older persons were in the sample of drivers studied, they made up a very small percentage (8 of 402) and differences were hard to substantiate.

More recently, Knoblauch, Nitzburg, Reinfurt, Council, Zegeer, and Popkin (1995) examined the lack of understanding associated with a variety of protected and permitted left-turn signal displays. They found that many drivers, both younger and older, do not understand the protected/permitted signal phasing, and they suggested that efforts to improve motorist comprehension of left-turn signal phasing should be targeted at the entire driving population. In focus group discussions, many older drivers reported that they avoid intersections that do not have a protected-only (left-turn arrow) phase or those where the time allowance for left turns was too short. In addition, the situation where the green arrow eventually turns to a solid green ball was generally confusing and not appreciated by the older participants. Among the recommendations made by the older drivers were:

• Provide as many protected left-turn opportunities as possible.
• Standardize the sequence for the left-turn green arrow so that it precedes solid green or red.
• Lengthen the protected left-turn signal.
• Lengthen the left-turn storage lanes so that turning traffic does not block through traffic.
• Make traffic signal displays more uniform across the United States, including the warning or amber phase.
• Standardize the position and size of signals.
• Provide traffic lights overhead and to the side at major intersections.
• Paint a yellow line in the pavement upstream of the signal in a manner that, if the driver has not reached the line before the light has turned yellow, he/she cannot make it through before the red light.
• Provide borders around lights to minimize glare from the sun.
• Eliminate decorations on signal heads, because they are often green and red and may be confusing near signal faces.

Numerous studies have found that: (1) protected left-turn control is the safest, with protected/permitted being less safe than protected, but safer than permitted (Fambro and Woods, 1981; Matthais and Upchurch, 1985; Curtis et al., 1988); and (2) transitions from protected-only operations to protected/permitted operations experience accident increases (Cottrell and Allen, 1982; Florida Section of Institute of Transportation Engineers, 1982; Cottrell, 1985; Warren, 1985; Agent, 1987). According to Fambro and Woods (1981), for every left-turn accident during a protected phase, 10 would have occurred without protection. Before-and-after studies where intersections were changed from protected to permitted control have shown four- to sevenfold increases in left-turn accidents (Florida Section of Institute of Transportation Engineers, 1982; Agent, 1987).

Williams, Ardekani, and Asante (1992) conducted a mail survey of 894 drivers in Texas to assess motorists' understanding of left-turn signal indications and accompanying auxiliary signs. Drivers older than age 65 had the highest percentage of incorrect responses (35 percent). Results of the various analyses are as follows: (1) the use of a green arrow for protected-only left turns produced better comprehension than the use of a circular green indication, even when the circular green indication was accompanied by an auxiliary sign; (2) for a five-section signal head configuration, the display of a green left-turn arrow in isolation produced better driver understanding than the simultaneous display of a circular red indication and a green left-turn arrow; (3) the LEFT TURN YIELD ON GREEN auxiliary sign was associated with the smallest percentage of incorrect responses, compared with the LEFT TURN ON GREEN AFTER YIELD sign, the PROTECTED LEFT ON GREEN sign, and the LEFT TURN SIGNAL sign; and (4) the percentage of incorrect responses was 50 percent lower in the presence of a circular red indication compared with a red arrow; the red arrow was often perceived to indicate that a driver may proceed with caution to make a permitted left turn.

In another study conducted by Curtis et al. (1988), it was found that the delaware flashing red arrow was not correctly answered by any subject. The incorrect responses indicated conservative interpretations of the signal displays which would probably be associated with delay and may also be related to rear-end collisions. Drivers interpreted the delaware signal as requiring a full stop before turning, because a red indication usually means "stop," even though the signal is meant to remind motorists to exercise caution but not necessarily to stop unless opposing through traffic is present. Hulbert, Beers, and Fowler (1979) found a significant difference in the percentage of drivers younger than age 49 versus those older than age 49 who chose the correct meaning of the red arrow display. Sixty-one percent of the drivers older than age 49 chose "no turning left" compared with 76 percent of those younger than age 49. Although other research has concluded that the left-turn arrow is more effective than the red ball in some left-turn situations in particular jurisdictions where special turn signals and exclusive turn lanes are provided (Noel, Gerbig, and Lakew, 1982), drivers of all ages will be better served if signal indications are consistent. Therefore, it is recommended that the use of the arrow be reserved for protected turning movements and the color red be reserved for circular indications to mean "stop."

Hawkins, Womak, and Mounce (1993) surveyed 1,745 drivers in Texas to evaluate driver comprehension of selected traffic control devices. The sample contained 88 drivers age 65 and older. Three alternative signs describing the left-turn decision rule were evaluated: (1) R10-9, PROTECTED LEFT ON GREEN ARROW (in the Texas MUTCD but not the national MUTCD); (2) R10-9a, PROTECTED LEFT ON GREEN (in the Texas MUTCD but not the national MUTCD); and (3) R10-12, LEFT TURN YIELD ON GREEN . The R10-12 sign did the best job of the signs in the survey informing the driver of a permitted left-turn condition, with 74.5 percent choosing the desirable response. Of those who responded incorrectly, 13.6 responded that they would wait for the green arrow, and 4.3 percent made the dangerous interpretation that the left turn was protected when the green ball was illuminated. Incorrect responses were more often made by drivers age 65 and older.

The decisional processes drawing upon working memory crucial to safe performance at intersections may be illustrated through a study of alternative strategies for presentation of left-turn traffic control messages (Staplin and Fisk, 1991). This study evaluated the effect of providing advance left-turn information to drivers who must decide whether or not they have the right-of-way to proceed with a protected turn at an intersection. Younger (mean age of 37) and older (mean age of 71) drivers were tested using slide animation to simulate dynamic approaches to intersection traffic control displays, with and without advance cueing of the "decision rule" (e.g., LEFT TURN MUST YIELD ON GREEN ) during the intersection approach. Without advance cueing, the decision rule was presented only on a sign mounted on the signal arm across the intersection as per standard practice, and thus was not legible until the driver actually reached the decision point for the turning maneuver. Cueing drivers with advance notice of the decision rule through a redundant upstream posting of sign elements significantly improved both the accuracy and latency of all drivers' decisions for a "go/no go" response upon reaching the intersection, and was of particular benefit to the older test subjects. Presumably, the benefit of upstream "priming" is derived from a reduction in the requirements for serial processing of concurrent information sources (sign message and signal condition) at the instant a maneuver decision must be completed and an action performed.

Stelmach, Goggin, and Garcia-Colera (1987) found that older adults were particularly impaired when preparation was not possible, showing disproportionate response slowing when compared with younger subjects. When subjects obtained full information about an upcoming response, reaction time (RT) was faster in all age groups. Stelmach et al. (1987) concluded that older drivers may be particularly disadvantaged when they are required to initiate a movement in which there is no opportunity to prepare a response. Preparatory intervals and length of precue viewing times are determining factors in age-related differences in movement preparation and planning (Goggin, Stelmach, and Amrhein, 1989). When preparatory intervals are manipulated in a way that older adults have longer stimulus exposure and longer intervals between stimuli, they profit from the longer inspection times by performing better and exhibiting less slowness of movement (Eisdorfer, 1975; Goggin et al., 1989). Since older drivers benefit from longer exposure to stimuli, Winter (1985) proposed that signs should be spaced farther apart to allow drivers enough time to view information and decide what action to take. Increased viewing time will reduce response uncertainty and decrease older drivers' RT.

The differences in maneuver decision responses demonstrated in the Staplin and Fisk (1991) study illustrate both the potential problems older drivers may experience at intersections due to working memory deficits, and the possibility that such consequences of normal aging can to some extent be ameliorated through improved engineering design practices. Staplin and Fisk (1991) also showed that older drivers had higher error rates and increased decision latencies for situations where the left turn was not protected. In particular, the most problematic displays were those with only one steady illuminated signal face (green ball) accompanied by a sign that indicated that it was not safe to proceed into the intersection with the assumption of right-of-way (LEFT TURN YIELD ON GREEN ). A correct response to this combination depends on the inhibition of previously learned "automatic" responses; a signal element with one behavior (go) was incorporated into a traffic control display requiring another, conflicting behavior.

Hummer, Montgomery, and Sinha (1991) evaluated leading and lagging signal sequences using a survey of licensed drivers in Indiana, an examination of traffic conflicts, an analysis of accident records, and a simulation model of traffic flow, to evaluate motorists' understanding and preference for leading and lagging schemes as well as determining the safety and delay associated with each scheme. Combinations of permitted and protected schemes included:
(1) protected-only/leading, in which the protected signal is given to vehicles turning left from a particular street before the green ball is given to the through movement on the same street; (2) protected-only/lagging, in which the green arrow is given to left-turning vehicles after the through movements have been serviced; (3) protected/permitted, in which protected left turns are made in the first cycle and a green-ball signal allows permitted turns later in the cycle; and (4) permitted/protected, in which permitted turns are allowed first in the cycle and protected left turns are accommodated later in the cycle. The protected-only/leading and protected/permitted schemes are known as "leading," and the protected-only/lagging and permitted/protected are known as "lagging" schemes. Of the 402 valid responses received, 248 respondents preferred the leading, 59 preferred the lagging sequence, and 95 expressed no preference. The most frequent reasons given for preference of the leading sequence were: it is more like normal; it results in less delay; and it is safer. There are apparent tradeoffs here, however; the leading sequence was associated with a higher conflict rate with pedestrians and a higher rate of run-the-red conflicts (drivers turning left during the clearance interval for opposing traffic), while the intersections with a lagging sequence were associated with a significantly higher rate of indecision conflicts than the leading intersections due to violations in driver expectancy. Overall, it is judged that consistency in signal phasing across intersections within a jurisdiction, as well as across jurisdictions, should be a priority, and that use of a leading protected left-turn phase offers the most benefits. A discussion of countermeasures for the protection of pedestrians may be found in the material that presents the Rationale and Supporting Evidence for Design Elements I and P.

Upchurch (1991) compared the relative safety of 5 types of left-turn phasing using Arizona Department of Transportation accident statistics for 523 intersection approaches, where all approaches had a separate left-turn lane, 329 approaches had 2 opposing lanes of traffic, and 194 approaches had 3 opposing lanes. The five types of left-turn phasing included (1) permitted, (2) leading protected/permitted, (3) lagging protected/permitted, (4) leading protected-only, and (5) lagging protected-only. For the 495 signalized intersections in the State highway system, most samples represented a 4-year accident history (1983­1986). For the 132 signalized intersections in 6 local jurisdictions in Arizona, samples ranged from 4 months to 4 years, all between 1981 and 1989. When the accident statistics were stratified by various ranges of left-turn volume and various ranges of opposing volume (vehicles per day), the following observations and conclusions were made for sample sizes greater than five, eliminating any conclusions about lagging protected-only phasing:

• Leading protected-only phasing had the lowest left-turn accident rate in almost every case. This was true in every left-turn volume range and every opposing volume range except one (19 out of 20 cases). Lagging protected/permitted was the exception for 3 opposing lanes and left-turn volumes of 0­1,000.
• When there were two lanes of opposing traffic, lagging protected/permitted tended to have the worst accident rate.
• When there were three lanes of opposing traffic, leading protected/permitted tended to have the worst accident rate.
• When there were two lanes of opposing traffic, the order of safety (accident rate from best to worst) was leading protected-only, permitted, leading protected/permitted, and lagging protected/permitted. However, there was a small difference in the accident rate among the last three types of phasing.
• When there were three lanes of opposing traffic, the order of safety (accident rate from best to worst) was leading protected-only, lagging protected/permitted, permitted, and leading protected/permitted.

Parsonson (1992) stated that a lagging left-turn phase should be used only if the bay provides sufficient storage, as any overflow of the bay during the preceding through-movement will spill into the adjacent through lane, blocking it. A lag should also be reserved for those situations in which opposing left-turn movements (or U-turns) are safe from the left-turn trap (or are prohibited). The "left-turn trap" occurs when the left-turning driver's right-of-way is terminated, while the opposing (oncoming) approach continues with a green arrow and an adjacent through movement. Thus, left-turning drivers facing a yellow indication are trapped; they believe that the opposing traffic will also have a yellow signal, allowing them to turn on the yellow or immediately after. Since the opposing traffic is not stopping, the turning driver is faced with a potentially hazardous situation. Locations where the left-turn trap is not a hazard include T-intersections, and those where the left turn (or U-turn) opposing the green arrow is prohibited or is allowed only on a green arrow (protected-only phasing). In addition, driver expectancy weighs heavily in favor of leading left turns, and driver confusion over lagging left turns results in losses in start-up time.

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I. Design Element: Traffic Control for Right-Turn/RTOR Movements at Signalized Intersections Table 12. Cross-references of related entries for right turn/RTOR movements at signalized intersections.

The right-turn-on-red (RTOR) maneuver provides increased capacity and operational efficiency at a low cost (Institute of Transportation Engineers [ITE], 1992). However, traffic control device violations and limited sight distances need to be addressed in order to reduce the potential for safety problems. ITE concluded that a significant proportion of drivers do not make a complete stop before executing an RTOR, and a significant portion of drivers do not yield to pedestrians. In a review of the literature on RTOR laws and motor vehicle crashes, Zador (1984) reported findings that linked RTOR to a 23 percent increase in all right-turning crashes, a 60 percent increase in pedestrian crashes, and a 100 percent increase in bicyclist crashes. Analysis of police accident reports in four States indicated that drivers who are stopped at a red light are looking left for a gap in traffic and do not see pedestrians and bicyclists coming from their right (Preusser, Leaf, DeBartolo, and Levy, 1982). Eldritch (1989) noted that, adding to the adverse effects RTOR has on pedestrian accidents, many motorists persist in making right turns on red even when there is a sign that prohibits the maneuver.

The most recent data available on the safety impact of RTOR were provided by Compton and Milton (1994) in a report to Congress by the National Highway Traffic Safety Administration. Using Fatal Accident Reporting System (FARS) data and data from four State files for 1989­1992, it was concluded that RTOR crashes represented a small proportion of the total number of traffic crashes in the four States (0.05 percent) and of all fatal (0.03 percent), injury (0.06 percent), and signalized-intersection crashes (0.40 percent). FARS data showed that approximately 84 fatal crashes per year occurred involving a right-turning vehicle at an intersection where RTOR is permitted; however, because the status of the traffic signal indication is not available in this database, the actual number of fatal crashes that occurred when the signal was red is not known. Slightly less than one-half of these crashes involved a pedestrian (44 percent), 10 percent involved a bicyclist, and 33 percent involved one vehicle striking another. Although no data on the age of the drivers involved in RTOR crashes were provided, there are reasons for concern that increasing problems with this maneuver may be observed with the dramatic growth in the number of older drivers in the United States.

The difficulties that older drivers are likely to experience making right turns at intersections are a function of their diminishing gap-judgment abilities, reduced neck/trunk flexibility, attention-sharing deficits, slower acceleration profile, and their general reduction in understanding traffic control devices compared with younger drivers. Right-turning drivers face possible conflicts with pedestrians, and restrictions in the visual attention of older drivers may allow pedestrian and vehicular traffic to go unnoticed. The fact that pedestrians may be crossing the side street, where they enter the path of the right-turning vehicle, places a burden upon the driver to search the right-turning path ahead. The result is the need to share attention between oncoming vehicles approaching from the left and pedestrians in the path to the right. Limitations in the range of visual attention, frequently referred to as "useful field of vision," further contribute to the difficulty of older drivers in detecting the presence of pedestrians or other vehicles near the driver's path. Older drivers, who may have greater difficulty maintaining rapid eye movements and associated head movements, are less likely to make correct judgments on the presence of pedestrians in a crosswalk or on their walking speed (Habib, 1980).

Researchers have identified that the right-turn maneuver is more problematic for older drivers compared with young or middle-aged drivers, presumably as a result of age-related diminished visual, cognitive, and physical capabilities. Knoblauch, Nitzburg, Reinfurt, Council, Zegeer, and Popkin (1995) conducted an analysis of right-angle, left-turning, right-turning, side swipe, and rear-end accidents at intersections in Minnesota and Illinois for the time period of 1985­1987, comparing accident proportions and characteristics of "middle-aged" drivers ages 30­50, "young-elderly" drivers ages 65­74, and "old-elderly" drivers age 75 and older. Turning right accounted for 35.8, 39.3, and 42.9 percent, respectively, of the middle-aged, young-elderly, and old-elderly drivers' accidents at urban locations. It appears that, for right-turning accidents, the middle-aged driver is most likely crossing the intersection on a green signal and the older drivers are turning right on a red signal in front of the oncoming middle-aged driver. Similar patterns emerged from examination of the rural signalized-intersection precrash maneuvers, with middle-aged drivers most often traveling straight, and older drivers most often turning left or right. Looking at the contributing factors in angle and turning collisions for both rural and urban signalized locations, the middle-aged group was much more likely to be characterized by the police officer as having exhibited "no improper driving." This occurred in 65 percent of the accidents involving this age group, compared with 30.7 percent of the young-elderly, and 13.4 percent of the old-elderly. The two elderly groups were more likely to be cited for failing to yield (42.0 percent of the old-elderly, 31.9 percent of the young-elderly, and 10.9 percent of the middle-aged); disregarding the traffic control device (30.7 percent of the old-elderly, 22.0 percent of the young-elderly, and 10.3 percent of the middle-aged); and driver inattention (8.2 percent of the old-elderly, 8.9 percent of the young-elderly, and 6.4 percent of the middle-aged).

Knowledge testing has indicated that, compared with younger drivers, older drivers are less familiar with the meaning of traffic control devices and relatively new traffic laws (McKnight, Simone, and Weidman, 1982). "Newness" of traffic laws, in this regard, relates not to the period of time that has elapsed since the device or law was implemented, but the low frequency with which drivers come in contact with the situation. Older drivers may not encounter right turn on red after stop (RTOR), no turn on red (NTOR), or red right-turn arrow situations on a daily basis, due to the significantly lower amount and frequency of driving in which they are engaged.

Hulbert, Beers, and Fowler (1979) found that when presented with a red arrow pointing right, only 75 percent of drivers across all age groups chose the correct answer (no right turn on red). There was a significant difference in the number of correct responses between drivers age 50 and older and drivers younger than age 50. The younger drivers gave the correct answer 80 percent of the time, whereas the older drivers were correct 66 percent of the time. Twenty-four percent of the older drivers thought they were permitted to turn right after coming to a stop, as did 14 percent of the drivers younger than age 50. Owolabi and Noel (1985) also determined that the right-turn red arrow is not a safe traffic control device. In their study, the red ball received significantly fewer violations than the arrow when used for right turns, regardless of the time of day.

Knoblauch et al. (1995) found that both drivers younger than the age of 65 and drivers age 65 and older failed to understand that they could turn right on a red ball after stopping in the right lane. Although the survey indicated that older drivers were more likely to stop and remain stopped (45 percent) than younger drivers (36 percent), the differences were not significant.

Staplin, Harkey, Lococo, and Tarawneh (1997) conducted a controlled field study to measure differences in drivers' RTOR behavior as a function of driver age and right-turn lane channelization. In this study, 100 subjects divided across 3 age groups were observed as they drove their own vehicles around test routes using the local street network in Arlington, Virginia. The three age groups were young/middle-aged (ages 25­45), young-old (ages 65­74), and old-old (age 75+). The percentage of drivers who made RTOR at the four intersections was included as a measure of mobility.

Staplin et al. (1997) found that significantly fewer drivers in the old-old driver group attempted to make an RTOR (16 percent), compared with young/middle-aged drivers (83 percent) and young-old drivers (45 percent). Similarly, young/middle-aged drivers made an RTOR nearly 80 percent of the time when they had the chance to do so, compared with nearly 36 percent for the young-old drivers and 15 percent for the old-old drivers. Drivers made significantly fewer RTOR's at the skewed channelized intersection than at the other three locations. Analysis of the percentage of drivers who made an RTOR without a complete stop showed that age, right-turn lane geometry, gender, and the age-by-geometry interaction were significant. Young/middle-aged drivers made an RTOR without a complete stop nearly 35 percent of the time, compared with nearly 25 percent for the young-old and 3 percent for the old-old drivers. Channelized intersections with or without exclusive acceleration lanes encouraged making an RTOR without a complete stop. The nonchannelized and the skewed locations showed the lowest percentage of RTOR's without a complete stop, and were not significantly different from each other. The three age groups showed significantly different performance. Old-old drivers almost always stopped before making an RTOR regardless of the right-turn lane geometry. In only 1 of 26 turns did an older driver not stop before making an RTOR; this occurred at the channelized right-turn lane with an acceleration lane. At the nonchannelized intersection (which was controlled by a STOP sign), 22 percent of the young/middle-aged drivers, 5 percent of the young-old drivers, and none of the old-old drivers performed an RTOR without a stop. Where an acceleration lane was available, 65 percent of the young/middle-aged drivers continued through without a complete stop, compared with 55 percent of the young-old drivers and 11 percent of the old-old drivers. The increased mobility exhibited by the younger drivers at the channelized right-turn lane locations (controlled by YIELD signs) was not exhibited by old-old drivers, who stopped in 19 of the 20 turns executed at the channelized locations. In summary, with increases in driver age, the likelihood of RTOR decreases to a very low level for the present cohort of old-old drivers, but when these individuals do engage in this behavior, they are virtually certain to come to a complete stop before initiating the maneuver. Therefore, the emphasis is to ensure adequate sight distance for the older turning driver, to provide sign and signal indications that are most easily understood by this group, and to prompt these motorists to devote adequate attention to pedestrians who may be in conflict with their turning maneuver.

Zegeer and Cynecki (1986) found that offsetting the stop bar—moving the stop bar of adjacent stopped vehicles back from the intersection by 1.8 to 3.0 m (6 to 10 ft)—was effective in providing better sight distance to the left for RTOR motorists. It also reduced the RTOR conflicts with other traffic and resulted in more RTOR vehicles making a full stop behind the stop bar. The offset stop bar was recommended as a countermeasure for consideration at RTOR-allowed sites that have two or more lanes on an approach and heavy truck or bus traffic, or unusual geometrics. It was also found that a novel sign (red ball with NO TURN ON RED, shown in figure 6) was more effective than the standard black-and-white NO TURN ON RED (R10-11a) sign, and should be added to the MUTCD. They offered that the red ball on the sign helps draw drivers' attention to it, particularly as intersections are associated with a preponderance of signs and information. Increasing the size of the standard NO TURN ON RED sign from its present size of 600 mm x 750 mm (24 in x 30 in) to 750 mm x 900 mm (30 in x 36 in) reduced the proportion of violations at most of the test sites. Finally, Zegeer and Cynecki (1986) found that an electronic NO TURN ON RED blank-out sign was found to be slightly better than the standard MUTCD sign in terms of reducing violations, and it was effective in increasing RTOR maneuvers when RTOR was appropriate, thereby reducing vehicle delay. Although the sign is more expensive than standard signs and pavement markings, the authors concluded it may be justified in situations where pedestrian protection is critical during certain periods (i.e., school zones) or during a portion of the signal cycle when a separate, opposing left-turn phase may conflict with an unsuspecting RTOR motorist.

Figure 6. Novel sign tested as a countermeasure by Zegeer and Cyneski, 1986.

Several studies have been conducted to determine whether regulatory signing aimed at turning motorists could reduce conflicts with pedestrians. Zegeer, Opiela, and Cynecki (1983) found that the regulatory sign YIELD TO PEDESTRIANS WHEN TURNING was effective in reducing conflicts between turning vehicles and pedestrians. They recommended that this sign be added to the MUTCD as an option for use at locations with a high number of pedestrian accidents involving turning vehicles. Zegeer and Cynecki (1986) found that the standard NO TURN ON RED sign with the supplementary WHEN PEDESTRIANS ARE PRESENT message was effective at several sites with low to moderate right-turn vehicle volumes. However, it was less effective when RTOR volumes were high. It was therefore recommended that the supplemental message WHEN PEDESTRIANS ARE PRESENT be added to the MUTCD as an accepted message that may be used with an NTOR sign when right-turn volume is light to moderate and pedestrian volumes are light or occur primarily during intermittent periods, such as in school zones. The supplemental message when added to the NTOR red ball sign reduced total pedestrian conflicts at one site and increased RTOR usage (as desired, from 5.7 percent to 17.4 percent), compared with full-RTOR prohibitions. It was recommended that the supplemental message be added to the MUTCD for the NTOR red ball sign, under low to moderate right-turn vehicle volumes and light or intermittent pedestrian volumes.

More recently, Abdulsattar, Tarawneh, and McCoy (1996) found that the TURNING TRAFFIC MUST YIELD TO PEDESTRIANS sign was effective in significantly reducing pedestrian-vehicle conflicts during right turns. The sign was installed at six marked crosswalks in Nebraska, where right-turn vehicle-pedestrian conflict data were collected before and after its installation in an observational field study. For the six study crosswalks combined, a conflict occurred in 51 percent of the observations in the before period, but in only 38 percent of the observations during the after period. The reductions in pedestrian-vehicle conflicts across the observation sites ranged from 15 to 30 percent, and were statistically significant.

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J. Design Element: Street-Name Signage

Table 13. Cross-references of related entries for street-name signage.

The MUTCD (1988) states that the lettering on street-name signs (D3) should be at least 100 mm (4 in) high. Burnham (1992) noted that the selection of letter size for any sign must evaluate the needs of the user, which are continuously changing as a function of changes in automotive technology, the roadway system, and the population itself. It is estimated that by the year 2020, 17 percent or more of the population—nearly one in five—will be older than 65 years of age (Transportation Research Board, 1988). The ability to read street signs is dependent on visual acuity as well as divided attention capabilities, both of which decline significantly with advancing age.

Older drivers participating in focus groups and completing questionnaires for traffic safety researchers over the past decade have consistently stated that larger street signs with bigger lettering and standardization of sign placement overhead would make driving an easier task (Yee, 1985; Gutman and Milstein, 1988; Cooper, 1990; Staplin, Lococo, and Sim, 1990; Benekohal, Resende, Shim, Michaels, and Weeks, 1992; Knoblauch, Nitzburg, Reinfurt, Council, Zegeer, and Popkin, 1995). Problems with placement included signs that were either obstructed by trees, telephone poles, billboards, or large trucks, or placed too close to or across the intersection rather than on the near side. Older drivers stated that they needed more advance notice regarding upcoming cross streets and larger street-name signs placed overhead, to give them more time to make decisions about where to turn. Also noted were difficulties reading traffic signs with too much information in too small an area, and/or with too small a typeface, which results in the need to slow down or stop to read and respond to the sign's message. May (1992) noted that providing sufficient time to allow motorists to make appropriate turning movements when approaching cross streets can improve safety and reduce congestion, and that consistent street signing across political jurisdictions can be helpful in this regard, as well as presenting an orderly, predictable picture of the region to tourists, businesspeople, and residents.

Taoka (1991) discussed "spare glance" duration in terms of how drivers allocate their visual search time among different tasks/stimuli. The tasks ranged from side/rearview mirror glances during turning to reading roadway name signs. Although specific results were not differentiated by age, Taoka asserted that 85th percentile glance times at signs (about 2.4 s) were likely too long, as 2.0 s is the maximum that a driver should divert from the basic driving task. Since older drivers are more apt to be those drivers taking longest to read signs, these results imply that they will commonly have problems dividing attention between searching for/reading signs and the basic driving task. Malfetti and Winter (1987) observed that older drivers exhibited excessive vehicle-braking behavior whenever a signal or road sign was sighted. This was categorized as an unsafe behavior, because it is confusing and disruptive to following traffic when the lead vehicle brakes for no apparent reason. These researchers obtained many descriptions of older drivers who stopped suddenly at unexpected times and in unexpected places, frequently either within the intersection or 12 m (40 ft) before the intersection to read street signs.

The visibility of retroreflective signs must be considered with regard to their dual requirements of detection and legibility. The sign components affecting detection are sign size, color, shape, and message or content design. External factors affecting sign detection include its placement (e.g., left, right, or overhead), the visual complexity of the area, and the contrast of the sign with its background. The component parts of retroreflective signs that determine legibility fall into two major classes of variables: character and message. Character variables include the variables related to brightness—i.e., contrast, luminance, color, and contrast orientation—as well as font, letter height, letter width, case, and stroke width. Message variables address the visibility issues of spacing and include interletter, interword, interline, and copy-to-border distances.

Most studies of sign legibility report legibility distance and the letter height of the stimulus; dividing the former measure by the latter defines the "legibility index" (LI), which can serve as a common denominator upon which to compare different studies. Forbes and Holmes (1939) used the LI to describe the relative legibility of different letter styles. Under daytime conditions, series B, C, and D were reported to have indexes of 0.4 m/mm, 0.5 m/mm, and 0.6 m/mm (33, 42.5, and 50 ft/in), respectively. Forbes, Moskowitz, and Morgan (1950) found the wider, series E letters to have an index of 0.66 m/mm (55 ft/in). Over time the value of 0.6 m/mm (50 ft/in) of letter height has become the nominal, though arbitrary and disputed, standard. Based on the physical attributes of the older driver population, the current standard of 50 ft of legibility for every 1 in of letter height (corresponding to a visual acuity of 20/25) exceeds the visual ability of approximately 40 percent of the drivers between ages 65 and 74. The LI is important to the size requirement determination for a sign in a specific application.

Mace (1988), in his work on minimum required visibility distance (MRVD) for highway signs, noted the following relationships:

Required letter size = MRVD / LI or Required LI = MRVD / letter size

Either the letter size or the LI may be manipulated to satisfy the MRVD requirement, which specifies the minimum distance at which a sign should be read for proper driver reaction.

Olson and Bernstein (1979) suggested that older drivers should not be expected to achieve an LI of 0.6 m/mm (50 ft/in) under most nighttime circumstances. The data provided by this report gives some expectation that 0.48 m/mm (40 ft/in) is a reasonable goal under most conditions. A 0.48 m/mm (40 ft/in) standard can generally be effective for older drivers, given contrast ratios greater than 5:1 (slightly higher for guide signs) and luminance greater than 10 cd/m² for partially reflectorized signs. With regard to the effect of driver age on legibility, Olson, Sivak, and Egan (1983) concluded that older drivers require more contrast between the message and the sign's background than younger drivers to achieve the same level of comprehension. They also noted that legibility losses with age are greater at low levels of background luminance. A reduction in legibility distance of 10 to 20 percent should be assumed when signs are not fully reflectorized. Also, higher surround luminance improved the legibility of signs more for older drivers and reduced the negative effects of excessive contrast. In general, the LI for older drivers is 70 to 77 percent that of younger drivers. The average LI for older drivers is clearly below the nominal value of 0.6 m/mm (50 ft/in) of letter height. The means for older drivers are generally between 0.48 m/mm and 0.6 m/mm (40 and 50 ft/in); however, the 85th percentile values reported are between 0.36 and 0.48 m/mm (30 and 40 ft/in) (Sivak, Olson, and Pastalan, 1981; Kuemmel, 1992; Mace, Garvey, and Heckard, 1994). Mace (1988) concluded that a most conservative standard would provide drivers with 2 minutes of arc, which corresponds to 20/40 vision and a 0.36 m/mm (30 ft/in) LI.

In a laboratory simulation study, Staplin et al. (1990) found that older drivers (ages 65­80) demonstrated a need for larger letter sizes to discern a message on a guide sign, compared with a group of younger drivers (ages 19­49). To read a one-word sign, older drivers required a mean letter size corresponding to 2.5 minutes of visual angle (or a Snellen acuity of 20/50), compared with the mean size required by younger drivers of 1.8 minutes of visual angle (or Snellen acuity of 20/35). Character size requirements increased for both age groups when the message contained four words, to 3.78 minutes of visual angle (acuity equivalent of 20/75) for the older drivers, and to 2.7 minutes of visual angle (acuity equivalent of 20/54) for the younger drivers. The main effect of age for the word and message legibility measure was highly significant. Staplin et al. (1990) concluded that for standard highway signing, an increase in character size in the range of 30 percent appears necessary to accommodate age-related acuity differences across the driving population.

Finally, the MUTCD states that street-name signs should be placed at least on diagonally opposite corners so that they will be on the far right-hand side of the intersection for traffic on the major street. It further states that on intersection approaches, a supplemental street-name sign may be erected separately or below an intersection-related warning sign, and when combined with a yellow diamond sign, the color should be a black message on a yellow background. Burnham (1992) noted that signs located over the highway are more likely to be seen before those located on either side of the highway. In this regard, Zwahlen (1989) examined detection distances of objects in the peripheral field versus line-of-sight detection and found that average detection distances decrease considerably as the peripheral visual detection angle increases. Placement of street-name signs overhead places the sign in the driver's forward line of sight, eliminating the need for the driver to take his/her eyes away from the driving scene, and reduces the visual complexity of the sign's surround, but under some sky conditions (e.g., backlit by the sun at dawn and dusk) the sign may be unreadable. Thus, overhead street-name signing should be a supplement to standard roadside signing.

Use of a supplemental street-name sign with an advance warning crossroad, side road, or T-intersection sign (W2-1, W2-2, W2-3, and W2-4) provides the benefit of additional decision and maneuver time if a lane change is required prior to reaching the intersection. Midblock street-name signing provides the same benefit. Phoenix, Arizona, a city with a large older driver population, has been using "jumbo" street-name signs since 1973. These signs are 400 mm (16 in) in height and use 200-mm (8-in) capital letters.

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K. Design Element: One-Way/Wrong-Way Signage

Table 14. Cross-references of related entries for one-way/wrong-way signage.

Vaswani (1974, 1977) found that approximately half of the incidents that involved wrong-way driving on multilane divided highways without access control occurred at intersections with freeway exits and with secondary roads. These wrong-way movements resulted from left-turning vehicles making a left turn into a lane on the near side of the median, rather than turning around the nose of the median into a lane on the far side. In an analysis of 96 accidents resulting from wrong-way movements on divided highways in Indiana from 1970 through 1972, Scifres and Loutzenheiser (1975) found that wrong-way movements most often occur under conditions of low traffic volume, low visibility, and low lane-use density. In addition, it was reported that 69 percent of the wrong-way drivers were drunk, older (age 65 and older), or fatigued (driving between 12 a.m. and 6 a.m.). A review of the literature by Crowley and Seguin (1986) reported that (1) there are significantly more incidents of wrong-way driving than there are accidents, and (2) drivers older than 60 years of age are overrepresented in wrong-way movements on a per-mile basis.

Further evidence of older driver difficulties likely to result in wrong-way movements was reported by McKnight and Urquijo (1993). These researchers examined 1,000 police forms that documented observations of incompetence when an older driver was either stopped for a violation or involved in a crash. They found that two of the primary behaviors that brought these drivers to the attention of police were driving the wrong way on a one-way street and driving on the wrong side of a two-way street. The drivers' mistakes contributed to many violations (149) but few accidents (29).

The ability to abstract information and make quick decisions about it are capabilities required to safely perform the driving task. Evidence has been found that older drivers' accidents often occur as the result of overly attending to irrelevant aspects of a driving scene (Planek and Fowler, 1971). Hasher and Zacks (1988) argued that older adults are deficient in inhibitory processes, and as a result, they frequently direct attention to irrelevant information at the expense of relevant information. The selective attention literature generally suggests that for adults of all ages, but particularly for the elderly, the most relevant information must be signaled in a dramatic manner to ensure that it receives a high priority for processing in situations where there is a great deal of complexity. Mace (1988) stated that age differences in glare sensitivity and restricted peripheral vision coupled with the process of selective attention may cause higher conspicuity thresholds for older drivers. Overall, these deficits point to the need for more effective and more conspicuous signs, realized through provision of multiple or advance signs as well as changes in size, luminance, or placement of signs.

The most comprehensive survey of current policies and practices for signing intersections to inform drivers of travel direction and to prevent wrong-way movements was conducted in the 48 contiguous States and in 35 of the largest cities by Crowley and Seguin (1986). They found considerable variability in the location, placement, and types of signs used to prevent wrong-way movements from occurring. The greatest variability in practice was reported in locations where a median divider exists. The study authors reported that median width is a key factor in the number, type, and location of signs to be used. When medians are extremely narrow, there appears to be little confusion that the intersecting roadway is two-way and drivers have less need for special signs to indicate travel direction. Where the median is sufficiently large, the intersection will be generally signed as two separate one-way roadways. A problem in defining what is "wide" and what is "narrow" was shown in the responses from a survey of practitioners across the United States, where there was a significant range in values around the 9 m (30 ft) delineation point specified by the MUTCD (para. 2A-31). The majority of jurisdictions tended to treat wide-median divided highways as if they were two separate intersections for the purpose of direction and turn-prohibition signing. The most commonly reported sign configuration implemented in the jurisdictions that responded to the survey was the MUTCD standard of a pair of ONE WAY signs (R6-1) on the near right-hand corners and far left-hand corners of each intersection with the directional roadway. A second pattern reported was a slight variation of the MUTCD standard, where the jurisdictions required a far-right sign (either a ONE WAY or a NO RIGHT TURN symbol sign) at the second intersection. Although many jurisdictions followed the MUTCD specifications for location of signs, many reported that they replaced a near-side ONE WAY sign with a NO RIGHT TURN sign (R3-1), even though the MUTCD states that the turn prohibition sign may be used to supplement the near-right/far-left pair of ONE WAY signs. The third pattern reported by some jurisdictions was to treat the divided highway, regardless of median width, as if it were a single intersection. In this case, a left/median sign for the first one-way roadway and a far-right sign for the second one-way roadway were considered sufficient. Where jurisdictions implement the third pattern, there was more emphasis on the use of the dIVIDED HIGHWAY CROSSING sign (R6-3) to supplement the limited amount of directional information. In one jurisdiction, signing was limited to the use of the dIVIDED HIGHWAY CROSSING sign.

Crowley and Seguin (1986) reported that some jurisdictions recommended the use of optional signs—i.e., DO NOT ENTER (R-5-1), WRONG WAY (R5-9), and KEEP RIGHT (R4-7)—but noted that these signs are not helpful to a motorist making decisions as he/she approaches an intersection; they are detected only when the driver begins a wrong-way movement upon reaching the intersection. In this regard, a number of jurisdictions reported that they required the use of the dIVIDED HIGHWAY CROSSING sign, as it is the only sign available that has a direct impact on the decision process of drivers approaching a divided highway with a median. The MUTCD states that this sign may be used as a supplemental sign on the approach legs of a roadway that intersects with a divided highway. Although this sign was not included in the set of traffic control devices tested by Hulbert and Fowler (1980), these researchers found that where complex driver judgments were required in conjunction with the use and understanding of particular driving situations, larger percentages of drivers failed to correctly respond to the meaning of traffic control devices. The comprehensibility of the dIVIDED HIGHWAY CROSSING sign has not been reliably documented.

Crowley and Seguin (1986) also conducted a laboratory study and a field validation study using subjects in three age groups (younger than age 25, ages 25­54, and age 55 and older) to identify signing practices that best provide information to minimize the possibility of wrong-way turning movements. Subjects were asked to identify driver actions that were either directly or by implication prohibited (by signs, markings, etc.), and to do so as quickly as possible. In the laboratory study, projected scenes of intersections containing a median (divided highway) were associated with higher error rates and longer decision latencies than scenes containing T-intersections and intersections of a two-way street with a one-way street (no median). The untreated intersections, where geometry alone was tested to determine the extent to which it conveyed an intrinsic "one-way" message, resulted in the worst performance; thus, any signing, regardless of the configuration, appears to be superior to no signing. However, even when the standard MUTCD near-right/far-left placement of ONE WAY signs were presented, large numbers of subjects did not recognize that the projected scene was that of a divided highway. Furthermore, the addition of a DIVIDED HIGHWAY CROSSING sign at the near-right corner of the intersection did not significantly reduce the overall error rate. Subjects age 55 and older had fewer correct responses and longer decision latencies than subjects in the two younger age groups. Field study results showed the following: (1) unsignalized divided highways resulted in more extreme steering patterns than signalized divided highways, at both of the one-way locations; (2) the use of ONE WAY signs in the left/median and far-right locations for medians as narrow as 6 m (20 ft) and as wide as 12.8 m (42 ft) showed superior performance to the single left/median ONE WAY sign; and (3) at undivided intersections of a two-way street with a one-way street, the most extreme variation in steering position was shown for the untreated intersections, suggesting that any signing treatment is better than none.

Crowley and Seguin (1986) noted that because there are intersections with specific physical factors that make the basic near-right/far-left rule inappropriate, the following text should be added to the MUTCD in section 2B-29 to bring the MUTCD and actual practice more in agreement and to reflect the actual manner in which the practitioner must respond to the problem of signing to prevent wrong-way traffic movements while providing positive guidance to drivers: "However, if an engineering study demonstrates the specified placements to be inappropriate due to factors such as sight distance restrictions, approach roadway grade and/or alignment, complex background, etc., one-way signs should be placed so as to provide the best possible guidance for the driver." In addition, a revision to section 2A-31 was proposed, which states that for medians of 9 m (30 ft) and under, both the left/median and far-right locations should be implemented when a divided highway justifies any form of one-way signing (see figure 7). DIVIDED HIGHWAY CROSSING, DO NOT ENTER, and WRONG WAY signs are optional, depending on the specific problem at a narrow median intersection. The authors note, however, that when a median is very narrow, one-way signing is usually unnecessary.

For medians greater than 9 m (30 ft), Crowley and Seguin (1986) suggested the use of ONE WAY signs posted at each of the following locations, for each direction of traffic: near right, median left, and far right. WRONG WAY and DO NOT ENTER signs are again optional. The resulting configuration is consistent with that shown earlier in Recommendation 4 of Design Element E.

Figure 7. Suggested revision of MUTCD Figure 2-3a, for medians
less than or equal to 9 m (30 ft). Source: Crowley and Seguin, 1986.

For four-way intersections (i.e., the intersection of a one-way street with a two-way street), the near-right/far-left locations were recommended by Crowley and Seguin (1986) regardless of whether there is left-to-right or right-to-left traffic. An additional ONE WAY sign located on the far-right side may be necessary in certain locations where approach grade and angle may direct the driver's field of view away from the "normal" sign locations (see figure 9).

Figure 8. Recommended location of ONE WAY signs for T-type intersections. Source:
Crowley and Seguin, 1986.

Finally, as noted in the "Rationale and Supporting Evidence" for Design Element E, the potential for wrong-way movements at intersections with channelized (positive) offset left-turn lanes (within a raised median) increases for the driver turning left from the minor road onto the major road, who must correctly identify the proper median opening into which he/she should turn. The following countermeasures were recommended at intersections with a divided median on the receiving leg, where the left-turn lane treatment results in channelized offset left-turn lanes (e.g., a parallel or tapered left-turn lane between two medians); these countermeasures are intended to reduce the potential for wrong-way maneuvers by drivers turning left from the stop-controlled minor roadway:

• Proper signing (advance dIVIDED HIGHWAY CROSSING signs, and proper positioning of WRONG WAY, DO NOT ENTER, and ONE WAY signing at the intersection) must be implemented.
• The channelized left-turn lanes should contain white painted lane-use arrow pavement markings (left-turn only).
• Pavement markings which scribe a path through the turn are recommended to reduce the likelihood for the wrong-way movement.
• The use of a wide (600-mm [24-in]) white stop bar is recommended at the end of the channelized left-turn lane as a countermeasure to aid in preventing a potential wrong-way movement. This countermeasure was found to be effective in preventing wrong-way entries onto freeway exit ramps in Georgia (Parsonson and Marks, 1979).
• Placement of 7-m (23.5-ft) wrong-way arrows in the through lanes is recommended, as specified in the MUTCD requirements for wrong-way traffic control for locations determined to have a special need, section 2E-40. Wrong-way arrows have been shown to reduce the frequency of wrong-way movements at freeway interchanges (Parsonson and Marks, 1979).
• Indistinct medians are considered to be design elements that tend to reduce a driver's ability to see and understand the overall physical and operational features of an intersection, increasing the frequency of wrong-way movements (Scifres and Loutzenheiser, 1975). Delineation of the median noses using reflectorized paint and other treatments will increase their visibility and should improve driver understanding of the intersection design and function.

The recommended placement of these traffic control devices was illustrated in Recommendation 4 of Design Element E.

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L. Design Element: Stop- and Yield-Controlled Intersection Signage

Table 15. Cross-references of related entries for stop- and yield-controlled intersection signage.

Mace and Pollack (1983) noted that conspicuity is not an observable characteristic of a sign but a construct which relates measures of perceptual performance with measures of background, motivation, and driver uncertainty. In this regard, conspicuity may be aided by multiple treatments or advance signing as well as changes in size, contrast, and placement. They noted that STOP signs following a STOP AHEAD (W3-1a) sign are more conspicuous not only to older drivers but to everyone, because expectancy has been increased.

The need for appropriate levels of brightness to ensure conspicuity and timely detection by drivers of highway signs, including STOP and YIELD signs, was addressed in FHWA-sponsored research to establish minimum retroreflectivity requirements for these devices. Mace developed a model to derive the retroreflectivity levels necessary for adequate visibility distance, taking into account driver age and visual performance level, as well as the driver's response requirements (action versus no action) to the information presented on a given sign when encountered in a given situation (city, highway) with an assumed operating speed (ranging from 16 km/h [10 mi/h] to 104 km/h [65 mi/h]), for signs of varying size and placement (shoulder, overhead). This work is reported by Ziskind, Mace, Staplin, Sim, and Lococo (1991), and subsequent guidelines have been promulgated by FHWA. Taking speed and sign application into account, the recommended retroreflectivity for STOP signs resulting from this research ranged between 10 cd/m² /lux up to 24 cd/m²/lux for the sign background (red) area, with significantly higher values for the sign legend. For the YIELD sign, the recommended levels ranged between 24 and 39 cd/m²/lux. These units express the sign brightness or luminance, measured in candelas per square meter, resulting from a given level of incident illumination, measured in lux. A retroreflectometer is used to obtain these data in the field. Because both the STOP and YIELD signs are so extensively overlearned by drivers, their comprehension is believed to be associated with the icon, i.e, their unique shape and coloration. Thus, the brightness of the sign's background area is most critical, because these devices will typically be recognized and understood as soon as they are detected.

Age-related deficits in vision and attention are key to developing recommendations for improved stop control and yield control at intersections. Researchers examining the State accident records of 53 older drivers found that those with restrictions in their useful field of view (UFOV)—a measure of selective attention and speed of visual processing—had 15 times more intersection accidents than those with normal visual attention (Owsley, Ball, Sloane, Roenker, and Bruni, 1991). A follow-up study with a sample of 300 drivers demonstrated that UFOV could account for up to 30 percent of the variance in intersection accident experience (Ball, Owsley, Sloane, Roenker, and Bruni, 1994). Additional relevant findings may be cited from a simulator study of peripheral visual field loss and driving impairment which also examined the actual driving records of the study participants (Szlyk, Severing, and Fishman, 1991). It was found that visual function factors, including acuity as well as visual field measures, could account for 26 percent of the variance in real-world accidents. Also, greater visual field loss was associated in the simulator data with greater distance traveled ("reaction distance") before responding to a peripheral stimulus (e.g., a STOP sign).

A considerable body of evidence exists documenting the difficulties of older driver populations in negotiating stop-controlled intersections. Specifically, analyses of accident and violation types at these sites highlight the older driver's difficulty in detecting, comprehending, and responding to signs within an appropriate timeframe for the safe completion of intersection maneuvers.

Statistics on Iowa fatal accidents show that during 1986­1990, running STOP signs was a contributing circumstance in 297 fatal accidents which killed 352 people; drivers age 65 and older accounted for 28 percent of the fatal crashes, and drivers younger than age 25 were involved in 27 percent of the fatal crashes (Iowa Department of Transportation, 1991). Stamatiadis, Taylor, and McKelvey (1991) found that at stop-controlled urban intersections, the percentage of drivers age 75 and older involved in right-angle accidents was more than double that of urban signalized intersections. Malfetti and Winter (1987), reporting on the unsafe driving performance of drivers age 55 and older, noted that older drivers frequently failed to respond properly or respond at all to road signs and signals; descriptions of their behavior included running red lights or STOP signs and rolling through STOP signs. Some older persons' behavior at STOP signs and signals seemed to indicate that they did not understand why they needed to wait when no other traffic was coming. Brainan (1980) used in-car observation to gain firsthand knowledge and insight into older people's driving behavior. Drivers in the 70 and older age group showed difficulty at two of the STOP signs on the test route; their errors were in failing to make complete stops, poor vehicle positioning at STOP signs, and jerky and abrupt stops. Campbell, Wolfe, Blower, Waller, Massie, and Ridella (1990), looking at police reports of crossing accidents at nonsignalized intersections, found that older drivers often stopped and then pulled out in front of oncoming traffic, whereas younger drivers more often failed to stop at all. Further evidence of unsafe behaviors by older drivers was provided in a study by McKnight and Urquijo (1993). Their data consisted of 1,000 police referral forms from the motor vehicle departments of California, Maryland, Massachusetts, Michigan, and Oregon; the forms included observations of incompetent behavior exhibited by older drivers who were stopped for a violation by law enforcement personnel or were involved in an accident. The specific behaviors contributing to the contact between the older driver and the police officer included failing to yield right-of-way or come to a complete stop at a STOP sign, and failing to stop or yield to other traffic; taken together, these behaviors contributed to significant numbers of accidents (74) and violations (114).

Data from 124,000 two-vehicle accidents (54,000 accidents at signalized intersections and 70,000 accidents at nonsignalized intersections) showed that drivers younger than age 25 and older than age 65 were overinvolved in accidents at both types of intersections (Stamatiadis et al. 1991). However, the overinvolvement of older drivers in nonsignalized intersection accidents was more pronounced than it was for signalized intersection accidents. Although the total number of accidents was reduced at nonsignalized intersections that contained signs when compared with unsigned intersections, the accident involvement ratios of older drivers were higher at signed intersections than at unsigned intersections. At nonsignalized intersections, the highest percentage of fatalities were the result of right-angle collisions (25 percent). In terms of the frequency of injury at nonsignalized intersections, rear-end accidents were the most frequent cause (35 percent), followed by right-angle accidents (18 percent), other-angle accidents (10 percent), and head-on/left-turn accidents (8 percent). The leading violation types for all older drivers in descending order were failure to yield right-of-way, following too closely, improper lane usage, and improper turning. At nonsignalized intersections, older drivers showed the highest accident frequency on major streets with two lanes in both directions (a condition most frequently associated with high-speed, low-volume rural roads), followed by roads with four lanes, and those with five lanes in both directions. These configurations were most often associated with low-speed, high-volume urban locations, where intersection negotiation involves more complex decisions involving more conflict vehicles and more visually distracting conditions.

Cooper (1990) utilized a database of all 1986 police-attended accidents in British Columbia, in an effort to compare the crash characteristics of older drivers with those of their younger counterparts. While 66.5 percent of crashes involving drivers ages 36­50 occurred at intersections, the percentage increased to 69.2 percent, 70.7 percent, and 76.0 percent for drivers ages 55­64, 65­74, and 75 and older, respectively. Overall, the two oldest groups identified in this analysis were significantly more accident involved at STOP/YIELD sign locations and less involved at either uncontrolled or signal-regulated locations. In follow-on questionnaires administered to a sample of drivers in each age group studied, intersection negotiation was mentioned by the older drivers as second in difficulty to problems changing lanes. About 20 percent of the older drivers mentioned not stopping properly at STOP signs. Vehicle maneuvering prior to the accident was a key variable for drivers over age 65, and in particular, for left turns at uncontrolled or STOP/YIELD sign-controlled intersections. Drivers ages 36­50 experienced only 10.9 percent of their accidents while turning left at this type of intersection, compared with 13.0, 15.4, and 19.5 percent of drivers ages 55­64, 65­74, and 75 and older, respectively.

Council and Zegeer (1992) conducted an analysis of intersection accidents occurring in Minnesota and Illinois for the time period of 1985­1987 to highlight accident types, situations, and causes of accidents, in an effort to increase the knowledge of how older drivers react at intersections. For all the analyses, comparisons were made between a "young-old" group (ages 65­74), an "old-old" group (age 75 or older), and a "middle-aged" comparison group (ages 30­50). Their findings regarding driver age differences in collision types, pre-accident maneuvers, and contributing factors are described below.

With respect to collision type at stop-controlled intersections, analysis of the data showed little difference in the proportion of crashes involving left-turning vehicles at either urban or rural locations when the older groups were compared with the middle-aged group. There was, however, a significant overinvolvement for both groups of older drivers in right-angle collisions, both in urban and in rural locations. At urban intersections, right-angle collisions accounted for 56.1 percent of the middle-aged driver accidents, compared with 64.7 percent of the young-old, and 68.3 percent of the old-old driver accidents. These percentages increase for all groups at rural intersections—61.3, 68.6, and 71.2 percent, respectively for middle-aged drivers, young-old drivers, and old-old drivers. Data for yield-controlled intersections showed older drivers overcontributing to left-turn collisions in urban areas and to angle collisions in both urban and rural areas.

Regarding pre-accident maneuvers at stop-controlled intersections, for both rural and urban locations, right-angle collisions were the most frequent collisions, and middle-aged drivers were more likely to be traveling straight or slowing/stopping than the two older groups. The older drivers were more likely to be turning left or starting from a stop than their younger counterparts. The pattern is similar for left-turning crashes. For rear-end collisions, the old-old drivers were more likely to be going straight (thus being the striking vehicle), and the middle-aged and young-old drivers were more likely to be stopped or slowing. For the few right-turning collisions at urban stop-controlled intersections, the middle-aged drivers were going straight and the old-old drivers were more likely to be turning left or right or starting from a stop. Rural stop-controlled locations showed the same patterns of precrash maneuvers among the three age groups.

Finally, breakdowns of contributing factors for the urban and rural stop-controlled intersections showed that the middle-aged drivers exhibited a higher proportion of no improper driving behavior, while the young-old and old-old drivers were more often cited for failure-to-yield, disregarding the STOP sign, and driver inattention. When starting from a stop, however, the old-old drivers had a lower probability of being cited for improper driving. When cited, the old-old group was more likely to have disregarded the STOP sign than the other two driver groups. The young-old drivers as well as the old-old drivers more frequently failed to yield than the middle-aged drivers.

For left turns, the middle-aged drivers again were more frequently found to have exhibited "no improper driving." The two older driver groups were most frequently cited with failure-to-yield. There was no difference in the number of drivers in each age group who disregarded the STOP sign. For going-straight situations, the middle-aged driver was found to have exhibited no improper driving behavior twice as often as the young-old driver and almost three times as often as the old-old driver. Failing to yield, disregarding the STOP sign, and inattention were most often cited as the contributing factor for the two older groups.

A two-way stop requires a driver to cross traffic streams from either direction; this poses a potential risk, because cross traffic may be proceeding rapidly and drivers may be less prepared to accommodate to errors made by crossing or turning drivers. Most critically, drivers proceeding straight through the intersection must be aware of the fact that the cross-street traffic does not stop, and that they must yield to cross-street vehicles from each direction before proceeding through the intersection. Older drivers are disproportionately penalized by the late realization of this operating condition, due to the various sources of response slowing noted earlier. Studies of cross-traffic signing to address this problem have shown qualified but promising results in a number of jurisdictions (Gattis, 1996). Although findings indicate that conversion of two-way to four-way stop operations may be more effective in reducing intersection accidents than the use of cross-traffic signing, there are obvious tradeoffs for capacity from this strategy. However, data from accident analyses in Arkansas, Oregon, and Florida reported by Gattis (1996) showed significant reductions in right-angle crashes after cross-traffic signing was installed at problem intersections. At this time there is no standard sign design to convey this message; Ligon, Carter, and McGee (1985) identified a number of alternate wordings used in different States. In addition, a warrant for use of a cross-traffic sign applied in the State of Illinois may be reviewed in the Gattis (1996) article.

The issue of driver expectancy, a key predictor of performance for older motorists, was addressed in a study by Agent (1979) to determine what treatments would make drivers more aware of a stop-ahead situation. Agent concluded that at rural sites, transverse pavement striping should be applied approximately 366 m (1,200 ft) in advance of the STOP sign to significantly reduce approach speeds. Later research (Agent, 1988) recommended the following operational improvements at intersections controlled by STOP signs: (1) installing additional advance warning signs; (2) modifying warning signs to provide additional notice; (3) adding stop bars to inform motorists of the proper location to stop, to obtain the maximum available sight distance; (4) installing rumble strips, transverse stripes, or post delineators on the stop approach to warn drivers that they would be required to stop; and (5) installing beacons. Although Agent emphasized that beacons do not eliminate the problem of drivers who disregard the STOP sign, flashing beacons used in conjunction with STOP signs at isolated intersections or intersections with restricted sight distance have been consistently shown to be effective in decreasing accidents by increasing driver awareness and decreasing approach speeds (California Department of Public Works, 1967; Cribbins and Walton, 1970; Goldblatt, 1977; King, Abramson, Cohen, and Wilkinson, 1978; Lyles, 1980).

With regard to the accident reduction effectiveness of rumble strips placed on intersection approaches, Harwood (1993) reported that rumble strips can provide a reduction of at least 50 percent in the types of accidents most susceptible to correction, including accidents involving running through a STOP sign. They can also be expected to reduce vehicle speed on intersection approaches and to increase driver compliance with STOP signs. In an evaluation conducted by the Virginia Department of Highways and Transportation (1981a) where rumble strips were installed at stop-controlled intersections, the total accident frequency was reduced by 37 percent, fatal accidents were reduced by 93 percent, injury accidents were reduced by 37 percent, and property-damage-only accidents were reduced by 25 percent. In this study, 39 of the 141 accidents in the before period were classified as being types susceptible to correction by rumble strip installation, particularly rear-end accidents and ran-STOP-sign accidents. The accident rate for these accident types was reduced by 89 percent. Carstens and Woo (1982) found that primary highway intersections where rumble strips were installed experienced a statistically significant reduction in the accident rate in the first year or two following their installation, both at four-way and T-intersections. The accident rate at the 21 study intersections decreased by 51 percent for total accidents and by 38 percent for ran-STOP-sign accidents. Carstens and Woo found no statistically significant change in accident rate at 88 intersections on secondary roads where rumble strips were installed. They concluded that rumble strips are more effective at primary highway intersections than secondary road intersections for the following reasons: (1) primary highways serve a higher proportion of drivers who are unfamiliar with the highway; (2) trips tend to be longer on primary highways so that fatigue and the monotony of driving may play a more important role than on secondary roads; (3) traffic volumes are higher on primary highways, so the number of potential conflicts is greater; and (4) the geometric layout of primary highway intersections is often more complex than that of secondary road intersections. These researchers also found that rumble strips may be more effective in reducing nighttime accidents at unlighted intersections than at lighted intersections. Harwood (1993) reported that several highway agencies commented that it was important to avoid the temptation to use rumble strips where they are not needed; if every intersection had rumble strips on its approach, rumble strips would soon lose their ability to focus the attention of the motorist on an unexpected hazard.

Before concluding this discussion, certain aspects of YIELD sign operations deserve mention. A YIELD sign facilitates traffic flow by preventing unnecessary stops and allowing drivers to enter the traffic flow with minimum disruption of through traffic. Most YIELD signs are posted where right-turning drivers can approach the cross street at an oblique angle. Such configurations benefit elderly drivers in carrying out the turning maneuver by avoiding the tight radii that characterize right-angle turns. However, in several respects, intersections regulated by YIELD signs place greater demands upon drivers than those employing other controls, in terms of gap selection, difficulty with head turning, lanekeeping, and maintaining or adjusting vehicle speed. The angle of approach to the street or highway being entered ranges from the near perpendicular to the near parallel. The closer the angle is to the parallel, the further the driver must turn his/her head to detect and to judge the speed and distance of vehicles on the road to be entered. Many elderly drivers are unable to turn their heads far enough to get a good look at approaching traffic, while the need to share attention with the road ahead necessarily limits the lane exposure to 1 or 2 s. Some drivers are reduced to attempting to judge distance and gaps by means of the outside mirror. The inability to judge gaps in this manner often results in the driver reaching the end of the access lane without having identified an appropriate gap. The driver in this situation comes to a complete stop and then must enter the cross street by accelerating from a stopped position. The difficulty in judging gaps may lead to aborted attempts to enter the roadway, leaving the older driver vulnerable to following drivers who direct their attention upstream and fail to notice that a vehicle has stopped in front of them. The need to share attention between two widely separated points results in eyes being off the intended path for lengthy periods. The diversion of attention, along with movement of the upper torso, hampers the older driver's ability to maintain directional control.

McGee and Blankenship (1989) report that intersections converted from stop to yield control are likely to experience an increase in accidents, especially at higher traffic volumes, at the rate of one additional accident every 2 years. In addition, converted yield-controlled intersections have a higher accident rate than established yield-controlled intersections. They note that while yield control has been found to be as safe as stop control at very low volumes, the safety impacts are not well established for higher volume levels. Agent and Deen (1975) reported that rural road accident types at yield-controlled intersections are different from those at stop-controlled intersections. At YIELD signs, more than half of the accidents were rear-end collisions, while more than half of the accidents at STOP signs were angle collisions.

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M. Design Element: Devices for Lane Assignment on Intersection Approach

Table 16. Cross-references of related entries for devices for lane assignment on intersection approach.

Older drivers' decreased contrast sensitivity, reduced useful field of view, increased decision time—particularly in response to unexpected events—and slower vehicle control during movement execution combine to put these highway users at greater accident risk when approaching and negotiating intersections. Contrast sensitivity and visual acuity are the visual/perceptual requirements necessary to detect pavement markings and symbols and to read lane control signs and verbal and symbolic pavement markings. The early detection of lane control devices, by cueing the driver in advance that designated lanes exist for turning and through maneuvers, promotes safer and more confident performance of any required lane changes. This is because the traffic density is lighter, there are more available gaps, and there are fewer potential conflicts with other vehicles and pedestrians the farther away from the intersection the maneuver is performed. Of course, even the brightest delineation and pavement markings will not be visible to an operator unless an adequate sight distance (determined by horizontal and vertical alignment) is available.

In an effort to analyze the needs and concerns of older drivers, the Illinois Department of Transportation sponsored a statewide survey of 664 drivers, followed up by focus group meetings held in rural and urban areas (Benekohal, Resende, Shim, Michaels, and Weeks, 1992). Within this sample, the following four age categories were used for statistical analyses: ages 66­68, ages 69­72, ages 73­76, and age 77 and older. Comparisons of responses from drivers ages 66­68 and age 77 and older showed that the older group had more difficulty following pavement markings, finding the beginning of the left-turn lane, driving across intersections, and driving during daytime. Similarly, the level of difficulty for reading street signs and making left turns at intersections increased with increasing driver age. Turning left at intersections was perceived as a complex driving task, made more difficult when channelization providing visual cues was absent and only pavement markings designated which lane ahead was a through lane and which was a turning lane. The processes of lane location, detection, and selection must be made upstream at a distance where a lane change can be performed safely. Late detection by older drivers will result in erratic maneuvers such as lane weaving close to the intersection (McKnight and Stewart, 1990).

More than half of the 81 older drivers participating in more recent focus group discussions stated that quite often they suddenly find themselves in the wrong lane, because (1) they have certain expectations about lane use derived from intersections encountered earlier on the same roadway, (2) the advance signing is inadequate or lacking, or (3) the pavement markings are covered by cars at the intersection (Staplin, Harkey, Lococo, and Tarawneh, 1997). The biggest problem with turn-only lanes reported by group participants was that there is not enough warning for this feature. The appropriate amount of advance notice, as specified by these drivers, ranged from 5 car lengths to 1.6 km (1 mi). Sixty-four percent of the participants said that multiple warning signs are necessary when the right lane becomes a turn-only lane, with the need for an initial sign 20 to 30 s away, and a second sign 10 s away from the turn location. The remaining participants said that these distances should be increased.

Even greater consensus was shown in this study regarding sign location for lane assignment. Seventy-nine percent of the group reported that overhead lane-use signs are far more effective than roadside-mounted signs for this type of warning. Several participants suggested that a combination of roadside and overhead signs, in addition to painted roadway markings, would be beneficial. Although painted roadway markings were deemed helpful, 84 percent of all participants stated that they are useless in isolation from signs, because they are usually at the intersection and are obscured by traffic, and they are frequently worn and faded. The result is that drivers end up in the wrong lane and must go in a direction they had not planned for, or they try to change lanes at a point where it is not safe to do so. Thus, a general conclusion from this study is that overhead signing posted in advance of, as well as at, an intersection provides the most useful information to drivers about movement regulations which may be difficult to obtain from painted arrows when traffic density is high or when pavement markings are obscured by snow or become faded, or where sight distance is limited.

In an early study conducted by Hoffman (1969), the installation of overhead lane-use control signs in advance of six intersections in Michigan contributed to a reduction in the total number of accidents by 44 percent in a 1-year period, and a reduction in the incidence of accidents caused by turning from the wrong lane by 58 percent. More recently, older drivers (as well as their younger counterparts) have been shown to benefit from redundant signing (Staplin and Fisk, 1991). In addition to redundant information about right-of-way movements at intersections, drivers should be forewarned about lane drops, shifts, and merges through advance warning signs, and ideally these conditions should not occur close to an intersection. Advance route or street signing as well as reassurance (confirmatory) signing/route marker assemblies across the intersection will aid drivers of all ages in deciding which lane will lead them to their destination, prior to reaching the intersection.

The MUTCD (sections 2B-17 and 2B-18) states that the standard size of lane-use control signs (R3-5 through R3-8) shall be 750 x 900 mm (30 x 36 in) when post-mounted, and when post-mounted lane-use control signs are used, one sign should be placed at the intersection and a second sign placed an adequate distance in advance of the intersection so that motorists can select the appropriate lane before waiting to reach the end of the line of waiting vehicles. It also states that overhead lane-use control signs are preferred because they can be placed over the lanes to which they apply. Section 2A-16 states that overhead sign installations should be illuminated where an engineering study shows that reflectorization will not perform effectively. The MUTCD further states that pavement markings may be used to supplement post-mounted signs and should be used with mandatory turn signs. With regard to pavement word and symbol markings, section 3B-20 specifies that large letters and numerals (2.4 m [8 ft] or more in height) should be used and that markings should be repeated in advance of mandatory turn lanes when necessary to prevent entrapment and to help motorists select the appropriate lane before reaching the end of the line of waiting vehicles. Although pavement markings have obvious limitations (e.g., limited durability when installed in areas exposed to heavy traffic, poor visibility on wet roads, and obscuration by snow in some regions), they have the advantage of presenting information to drivers without distracting their attention from the roadway.

Finally, the Institute of Transportation Engineers identified several features to enhance the operation of urban arterial trap lanes (through lanes that terminate in an unshadowed mandatory left- or right-turn regulation): (1) signing that gives prominent advance notice of the unexpected mandatory turn regulation, followed by a regulatory sign at the point where the mandatory turn regulation takes effect, followed by a third sign at the intersection itself if there are intervening driveways from which motorists might enter the lane; (2) supplemental pavement markings which consist of a double-width broken lane line beginning at the advance warning sign and extending to the first regulatory sign; (3) a pavement legend in the trap lane; and (4) overhead signing. Candidates for these remediations include left-turn trap lanes on roadways with high volumes, high speeds, poor approach visibility, and complex geometrics (Foxen, 1986).

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N. Design Element: Traffic Signal Performance Issues

Table 17. Cross-references of related entries for traffic signal performance issues.

Traffic signals are power-operated signal displays used to regulate or warn traffic. They include displays for intersection control, flashing beacons, lane-directional signals, ramp-metering signals, pedestrian signals, railroad-crossing signals, and similar devices. Warrants for traffic signals are thoroughly described in the MUTCD. The decision to install a traffic signal is based on an investigation of physical and traffic flow conditions and data, including traffic volume, approach travel speeds, physical condition diagrams, accident history, and gap and delay information (Wilshire, 1992). The MUTCD incorporates the intensity, light distribution, and chromaticity standards from the following Institute of Transportation Engineers (ITE) standards for traffic signals: Vehicle Control Signal Heads, ITE Standard No ST-008B (ITE, 1985b); Pedestrian Traffic Control Signal Indications, ITE Standard No. ST-011B (ITE, 1985a); Traffic Signal Lamps, ITE Standard No. ST-010 (ITE, 1986); and Lane-Use Traffic Control Signal Heads (ITE, 1980). Standards for traffic signals are important because it is imperative that they attract the attention of virtually every driver, including older drivers and those with impaired vision who meet legal requirements, as well as those who are fatigued or distracted, or who are not expecting to encounter a signal at a particular location. It is also necessary for traffic signals to function under a wide range of conditions including day and night, adverse weather, and visually complex surrounds.

To date, studies of traffic signal performance have not typically included observer age as an independent variable. Available evidence suggests, however, that older individuals have reduced levels of sensitivity to intensity and contrast, but not to color. Fisher (1969) reported that as a person ages, the ocular media yellows and has the effect of enhancing the contrast between a red signal and a sky background. However, this effect is more than offset by increasing light scatter within the eye, which diminishes contrast. Older drivers need increased levels of signal luminance and contrast in certain situations to perceive traffic signals as efficiently as 20- to 25-year-old drivers; however, higher signal intensities may cause disability glare. Fisher and Cole (1974), using data from Blackwell (1970), suggested that older drivers may require 1.5 times the intensity at 50 years of age and 3 times the intensity at 70 years of age, and protanopes (individuals with a color-vision deficiency resulting in partial or full insensitivity to red light) may require a fourfold increase. They noted that while increased intensity will ensure that older observers see the signal, the reaction time of older drivers will be longer than for younger drivers. To compensate for this, it would appear necessary to assume a longer required visibility distance, which would result in an increase in the signal intensity required. However, Fisher (1969) also suggested that no increase in signal intensity is likely to compensate for increasing reaction time with age. It therefore deserves emphasis that the goal of increased response times for older drivers, requiring longer visibility distances, can also be provided by ensuring that the available signal strength (peak intensity) is maintained through a wide, versus a narrow, viewing angle. This makes signal information more accessible over longer intervals.

It is generally agreed that the visibility issues associated with circular signals relate to the following factors: minimum daytime intensity, intensity distribution, size, nighttime intensity, color of signals, backplates, depreciation (light loss due to lamp wear and dirt on lenses), and phantom (apparent illumination of a signal in a facing sun). To place this discussion in context, it should also be noted that traffic signal recommendations for different sizes, colors, and in-service requirements have, in large part, been derived analytically from one research study conducted by Cole and Brown (1966).

In establishing minimum daytime intensity levels for (circular) traffic signals, the two driver characteristics that are considered with regard to the need to adjust peak intensity requirements are color anomalies and driver age. Cole and Brown (1968) determined that the optimum red signal intensity is 200 cd for a sky luminance of 10,000 cd/m², and an adequate signal intensity for this condition would be 100 cd. "Optimum" is defined by Cole and Brown (1968) to be the intensity that produces minimum reaction time plus 0.1 s. An "adequate" signal is one that is still not likely to be missed, although driver reaction time will be slower than for a signal of optimum intensity.

The number of organizations that specify a minimum standard for peak daytime traffic signal intensity is larger than the number of research studies upon which those standards are based. In fact, all of the standards including those for 200-mm (8-in) and 300-mm (12-in) signals, those for red, yellow, and green signals, and those for new and in-service applications are derived from a single requirement for a red traffic signal, established from the work of Cole and Brown (1966). The conclusion of this laboratory study was that a red signal with an intensity of 200 cd should invoke a certain and rapid response from an observer viewing the signal at distances up to 100 m (328 ft) even under extremely bright ambient conditions. This conclusion was based on experiments in which the background luminance was 5,142 cd/m². The results were linearly extrapolated to a background luminance of 10,000 cd/m² which yielded the 200 cd recommendation. Janoff (1990) concluded that a value of 200 cd minimum intensity for a red signal will suffice for observation distances up to 100 m (328 ft) and vehicle speeds up to 80 km/h (50 mi/h), based on analytic, laboratory, and controlled field experiments performed by Adrian (1963); Boisson and Pages (1964); Rutley, Christie, and Fisher (1965); Jainski and Schmidt-Clausen (1967); Cole and Brown (1968); Fisher (1969); and Fisher and Cole (1974). Fisher and Cole (1974) cautioned against using a value less than 200 cd, to ensure that older drivers and drivers with abnormal color vision will see the signal with certainty and with "reasonable speed."

For green signals, Fisher and Cole (1974) indicated that the ratio of green to red intensity should be 1.33:1, based on laboratory and controlled field research by Adrian (1963), Rutley et al. (1965), Jainski and Schmidt-Clausen (1967), and Fisher (1969), and the ratio of yellow to red should be 3:1, based on research performed by Rutley et al. (1965) and Jainski and Schmidt-Clausen (1967). Janoff (1990) noted that the evidence to support these ratios is somewhat variable, and support of these recommendations is mixed. Table 18, from Janoff (1990), presents the peak intensity requirements of red, green, and yellow traffic signals for 200-mm (8-in) signals for normal-speed roads and for 300-mm (12-in) signals for high-speed roads; the values presented exclude the use of backplates and ignore depreciation. A normal-speed road, in this context, includes speeds up to 80 km/h (50 mi/h), distances up to 100 m (328 ft), and sky luminances up to 10,000 cd/m². A high-speed road is defined as one with speeds up to 100 km/h (62 mi/h), distances up to 240 m (787 ft), and sky luminances up to 10,000 cd/m². Janoff also noted that although signal size is included, research performed by Cole and Brown (1968) indicated that signal size is not important because traffic signals are point sources rather than area sources and only intensity affects visibility. Thus, the required intensity can be obtained by methods other than increasing signal size (i.e., by using higher intensity sources in 200-mm signals).

Table 18. Peak (minimum) daytime intensity requirement (cd)
for maintained signals with no backplate. Source: Janoff, 1990.

Only two research reports provide intensity requirements for green and/or yellow signals based upon empirical data. Adrian (1963) used a subjective scale and threshold detection criteria in a study that tested red and green signals at different background luminances. He concluded that the intensity requirements for green were 1.0 and 1.2 times that of red for the subjective and threshold studies, respectively. Jainski and Schmidt-Clausen (1967) tested the ability of observers to detect the presence of a red, amber, or green spot, which was either 2 minutes or 1 degree, against varying background luminances. Their results found that green required 1.0 and 2.5 times that of red, and yellow required 2.5 and 3.0 times that of red, for 1 degree and 2 minutes, respectively. Using these results, most standards set requirements for green and yellow to be 1.3 and 3.0 times that of red, respectively. The CIE standard discusses the fact that the ratios of 1.3 and 3.0 for green and yellow appear to reflect the differences in the transmissivity of the varying color lenses.

The intensity distributions of traffic signals can be compared in two ways: percentage of peak or absolute value. For 200-mm (8-in) signals, the horizontal and vertical distributions between standards are generally similar, more so for the horizontal. The ITe distribution for 300-mm (12-in) signals is exactly the same as the ITE 200-mm (8-in) distribution. The CIE 300-mm distribution is very different from its 200-mm counterpart. The 300-mm CIe distribution requires a higher concentration of intensity in the center of the beam. The standards describe their requirements in the form of tables, which list the horizontal and vertical angular positions where a certain percentage of the peak intensity is required. Logically, maintaining a higher percentage of peak intensity for a given distance away from the center of the beam (line of sight) produces a larger angular area in which the signal indication may be perceived.

Two studies have been conducted for 200-mm (8-in) signals dealing with intensity distribution: an analytic one by Cole (1966), where the signal was in the observer's line of sight, and a controlled field experiment by Fisher (1969), where the signal was placed off the line of sight of the observer. Fisher's work is considered an extension of Cole's. Cole's work yielded an optimal distribution for traffic signals. Hulscher (1974) conducted photometric and analytic research to extend Cole's results to include 300-mm (12-in) signals. The optimum distribution for 200-mm (8-in) signals derived by Fisher (1969) is provided in figure 10, with and without a backplate. In Fisher's 1969 controlled field study, it was found that the use of a backplate permits the distribution of intensity to be more concentrated in the center of the beam. Figure 10 also presents the optimum distribution of 300-mm (12-in) signals as determined by Hulscher (1974), which was included in the 1980 CIE standard.

Some research has indicated that the dimming of signals at night may have advantages, while also reducing power consumption. Freedman, Davit, Staplin, and Breton (1985) conducted a laboratory study and controlled and observational field studies to determine the operational, safety, and economic impact of dimming traffic signals at night. Results indicated that drivers behaved safely and efficiently when signals were dimmed to as low as 30 percent of ITE recommendations. Previously, however, Lunenfeld (1977) cited the considerable range of night background luminances that may occur in concluding that in some brightly lit urban conditions, or where there is considerable visual noise, daytime signal brightness is needed to maintain an acceptable contrast ratio. The ITE standard presently does not differentiate between day and night intensity requirements. The CIE recommends that intensities greater than 200 cd or less than 25 cd be avoided at night and advises a range of 50 to 100 cd for night, except for high-speed roads where the daytime values are preferred. The South African and Australian standards allow for dimming but do not recommend an intensity level. While the option for dimming on a location-by-location basis should not be excluded, from the standpoint of older driver needs, there is no compelling reason to recommend widespread reduction of traffic signal intensity during nighttime operations.

It is common practice to try to enhance the visibility of signals by placing a large, black surround behind the signals. The backplate, rather than the sky, becomes the background of the signals, enhancing the contrast. The CIE (1988) recommends that all signals use backplates of a size (width) of three times the diameter of the signal. The ITE standard does not provide a backplate recommendation. In laboratory research, Cole and Brown (1966) found that the use of backplates reduces the required intensity by about 25 percent at distances of approximately 100 m (328 ft), where the luminance of the sky is 10,000 cd/m² and the speed is 80 km/h (50 mi/h), but it has little effect at longer distances unless the size of the backplate is excessive. Fisher (1969) and Fisher and Cole (1974) derived a 40-percent intensity reduction for 200-mm (8-in) signals at distances of 100 m (328 ft) and greater reductions at shorter distances, ranging up to 90 percent at distances under 25 m (82 ft). For practical purposes, a backplate three times the width of the signal reduces the intensity requirement by about 0.6 for distances up to 100 m (328 ft). In analytic computations based on Cole's work, Hulscher (1975) determined that a 300-mm (12-in) signal with a backplate requires approximately one-third less intensity.

As a practical matter, the use of a backplate would, in most cases, compensate for the effects of depreciation, since a backplate reduces the required intensity by roughly 25 percent while depreciation increases the requirement by the same amount. New guidelines published by the CIE (1988) suggest including an allowance of 25-percent transmissivity for depreciation due to dirt and aging (a 33-percent increase in intensity). The 200-cd requirement for red signals, as noted earlier, must be met after the depreciation factor has been taken into account.

A final issue with respect to signal performance and older drivers is the change intervals between phases, and the assumptions about perception-reaction time (PRT) on which these calculations are based. At present, a value of 1.0 s is assumed to compute change intervals for traffic signals, a value which, according to Tarawneh (1991), dates back to a 1934 Massachusetts Institute of Technology study on brake-reaction time. Tarawneh examined findings published by proponents of both "parallel" and "sequential" (serial) models of driver information processing, seeking to determine the best estimator for older individuals of a PRT encompassing six different component processing operations: (1) latency time (onset of stimulus to beginning of eye movement toward signal); (2) eye/head movement time to fixate on the signal; (3) fixation time to get enough information to identify the stimulus; (4) recognition time (interpret signal display in terms of possible courses of action); (5) decision time to select the best response in the situation; and (6) limb movement time to accomplish the appropriate steering and brake/accelerator movements.

Tarawneh's (1991) review produced several conclusions. First, the situation of a signal change at an intersection is among the most extreme, in terms of both the information-processing demand and subjective feelings of stress that will be experienced by many older drivers. Second, the most reasonable interpretation of research to date indicates that the best "mental model" to describe and predict how drivers respond in this context includes a mix of concurrent and serial-and-contingent information-processing operations. In this approach, the most valid PRT estimator will fall between the bounds of values derived from the competing models thus far, also taking into account age-related response slowing for recognition, decisionmaking, and limb movement. After a tabular summary of the specific component values upon which he based his calculations, Tarawneh (1991) called for an increase in the current PRT value used to calculate the length of the yellow interval (derived from tests of much younger subjects) from 1.0 s to 1.5 s to accommodate older drivers.

A contrasting set of results was obtained in a recent FHWA-sponsored study of traffic operations control for older drivers (Knoblauch, Nitzburg, Reinfurt, Council, Zegeer, and Popkin, 1995). This study compared the decision/response times and deceleration characteristics of older drivers (ages 60­71 and older) with those of younger drivers (younger than age 60) at the onset of the amber signal phase. Testing was conducted using a controlled field test facility, where subjects drove their own vehicles. Subjects were asked to maintain speeds of 48 km/h (30 mi/h) and 32 km/h (20 mi/h) for certain test circuits. The duration of the yellow signal was 3.0 s before turning to red. On half of the trials, the signal changed from green to yellow when the subject was 3.0 to 3.9 s from the signal, and on the remaining trials, when the subject was 4.0 to 4.9 s away from the signal. For three of the circuits, subjects were asked to brake as they normally would and to stop before reaching the intersection, if they chose to do so. During a fourth circuit, they were asked to brake to a stop, if they possibly could, if the light changed from green to yellow. Response times were measured for the drivers who stopped, from the onset of the yellow phase to the time the brake was applied.

Results of the Knoblauch et al. (1995) study showed no significant differences in 85th percentile decision/response times between younger and older drivers when subjects were close to the signal at either approach speed. The 85th percentile decision time of younger subjects was 0.39 s at 32 km/h (20 mi/h) and 0.45 s at 48 km/h (30 mi/h). For older drivers, these times were 0.51 and 0.53 s, for 32 km/h and 48 km/h (20 mi/h and 30 mi/h), respectively. When subjects were further from the signal at amber onset, older drivers had significantly longer decision/response times (1.38 s at 32 km/h [20 mi/h] and 0.88 s at 48 km/h [30 mi/h]) than the younger drivers (0.50 s at 32 km/h [20 mi/h] and 0.46 s at 48 km/h [30 mi/h]). The authors suggested that the significant differences between older and younger drivers occurred when the subjects were relatively far from the signal, and that some older subjects will take longer to react and respond when additional time is available for them to do so. Thus, they concluded that the older drivers were not necessarily reacting inappropriately to the signal. In terms of deceleration rates, there were no significant differences, either in the mean or 15th percentile values, between the older and younger subjects. Together, these findings led the authors to conclude that no changes in amber signal phase timing are required to accommodate older drivers.

Taking the review and study findings of Tarawneh (1991) and Knoblauch et al. (1995) into consideration, an approach that retains the 1.0-s PRT value for calculating the yellow change interval but acknowledges the significant body of work documenting age-related increases in PRT, especially where there is response uncertainty, appears most reasonable. A recommendation for an all-red clearance interval logically follows, with length determined according to the ITE (1992).

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O. Design Element: Fixed Lighting Installations

Table 19. Cross-references of related entries for fixed lighting installations.

The link between reduced visibility and highway safety, though it may be difficult to quantify in a cost benefit analysis, is conceptually straightforward. Low luminance contributes to a reduction in visual capabilities such as acuity, distance judgment, speed of seeing, color discrimination, and glare tolerance, which are already diminished capabilities in older drivers.

The Commission Internationale de l'Éclairage (1990) reports that road accidents at night are disproportionately higher in number and severity compared with accidents during the daytime. Data from 13 Organisation for Economic Co-operation and Development countries showed that the proportion of fatal nighttime accidents ranged between 25 and 59 percent (average value of 48.5 percent). In this evaluation of 62 lighting and accident studies, 85 percent of the results showed lighting to be beneficial, with approximately one-third of the results statistically significant.

In 1990, drivers (without regard to age) in the United States experienced 10.37 fatal involvements per 161 million km (100 million mi) at night and 2.25 during the day (Massie and Campbell, 1993). In their analysis, the difference between daytime and nighttime fatal rates was found to be more pronounced among younger age groups than among older ones, with drivers ages 20­24 showing a nighttime rate that was 6.1 times the daytime rate, and drivers age 75 and older showing a nighttime rate only 1.1 times the daytime rate. The lower percentage of nighttime accidents of older drivers may be due to a number of factors, including reduced exposure—older drivers as a group drive less at night—and a self-regulation process whereby those who do drive at night are the most fit and capable to perform all functional requirements of the driving task (National Highway Traffic Safety Administration, 1987).

A specific driving error with high potential for crash involvement is wrong-way movements. Analyses of wrong-way movements at intersections frequently associate these driving errors with low visibility and restricted sight distance (Vaswani, 1974, 1977; Scifres and Loutzenheiser, 1975) and note that designs that increase drivers' visibility and perception of access points to divided highways have been found to reduce the potential for accidents.

Inadequate night visibility, where the vehicle's headlights are the driver's primary light source, was reported by Vaswani (1977) as a factor that makes it more difficult for drivers to determine the correct routing at intersections with divided highways. Similarly, Woods, Rowan, and Johnson (1970) reported that locations where highway structures, land use, natural growth, or poor lighting conditions reduce the principal sources of information concerning the geometry and pavement markings are associated with higher occurrences of wrong-way maneuvers. Crowley and Seguin (1986) reported that drivers over the age of 60 are excessively involved in wrong-way movements on a per-mile basis. Suggested countermeasures include increased use of fixed lighting installations. Lighting provides a particular benefit to older drivers by increasing expectancy of needed vehicle control actions, at longer preview distances. It has been documented extensively in this Handbook that an older driver's ability to safely execute a planned action is not significantly worse than that of a younger driver. The importance of fixed lighting at intersections for older drivers can therefore be understood in terms of both the diminished visual capabilities of this group and their special needs to prepare farther in advance for unusual or unexpected aspects of intersection operations or geometry. Targets that are especially critical in this regard include shifting lane alignments; changing lane assignments (e.g., when a through lane changes to turn-only operation); a pavement width transition, particularly a reduction across the intersection; and, of course, pedestrians.

Detectability of a pedestrian is generally influenced by contrast, motion, color, and size (Robertson, Berger, and Pain, 1977). If a pedestrian is walking at night and does not have good contrast, color contrast, or size relative to other road objects, an increase in contrast will significantly improve his/her detectability. This is one effect of street lighting. Extreme contrasts as well as dark spots are reduced, giving the driver and the pedestrian a more "even" visual field. The effectiveness of fixed lighting in improving the detectability of pedestrians has been reported by Pegrum (1972); Freedman, Janoff, Koth, and McCunney (1975); Polus and Katz (1978); and Zegeer (1991).

While age-related changes in glare susceptibility and contrast threshold are currently accounted for in lighting design criteria, there are other visual effects of aging that are currently excluded from visibility criteria. Solid documentation exists of age-related declines in ocular transmittance (the total amount of light reaching the retina), particularly for the shorter wavelengths (cf. Ruddock, 1965); this suggests a potential benefit to older drivers of the "yellow tint" of high-pressure sodium highway lighting installations. The older eye experiences exaggerated intraocular scatter of light—all light, independent of wavelength (Wooten and Geri, 1987)—making these drivers more susceptible to glare. Diminished capability for visual accommodation makes it harder for older observers to focus on objects at different distances. Pupil size is reduced among older individuals through senile miosis (Owsley, 1987), which is most detrimental at night because the reduction in light entering the eye compounds the problem of light lost via the ocular media, as described above.

The loss of static and dynamic acuity—the ability to detect fine detail in stationary and moving targets—with advancing age is widely understood. Although there are pronounced individual differences in the amount of age-related reduction in static visual acuity, Owsley (1987) indicated that a loss of about 70 percent in this capability by age 85 is normal. Among other things, declines in acuity can be used to predict the distance at which text of varying size can be read on highway signs (Kline and Fuchs, 1993), under a given set of viewing conditions.

There are a number of other aspects of vision and visual attention that relate to driving. In particular, saccadic fixation, useful field of view, detection of motion in depth, and detection of angular movement have been shown to be correlated with driving performance (see Bailey and Sheedy, 1988, for a review). As a group, however, these visual functions do not appear to have strong implications for highway lighting practice, with the possible exception of the useful field of view. It could be argued that it would be advantageous to provide wider angle lighting coverage to increase the total field of view of older drivers. High-mast lighting systems can increase the field of view from 30 degrees to about 105 degrees (Hans, 1993). Such wide angles of coverage might have advantages for older drivers in terms of peripheral object detection. However, the disadvantages may outweigh the potential advantages of increasing the field of view. Although effective high-mast systems have been demonstrated (Ketvirtis and Moonah, 1995), high-mast lighting systems tend to sacrifice target contrast for increased field of view. Also, the increased visual clutter produced by the higher luminance levels in the periphery may contribute to attentional problems, particularly attention switching, that has been linked to accidents in older drivers (Summala and Mikkola, 1994). Thus, field of view is not considered as a parameter that needs to be optimized in lighting system design, especially when, with current technology, it is inevitable that visibility (i.e., target contrast) will be sacrificed.

Rockwell, Hungerford, and Balasubramanian (1976) studied the performance of drivers approaching four intersection treatments, differentiated in terms of special reflectorized delineators and signs versus illumination. A significant finding from observing 168 test approaches was that the use of roadway lighting significantly improved driving performance and earlier detection of the intersection, compared with the other treatments (e.g., signing, delineation, and new pavement markings), which showed smaller improvements in performance.

Finally, it must be emphasized that the effectiveness of intersection lighting depends upon a continuing program of monitoring and maintenance by the local authority. Guidelines published by AASHTO (1984) identify depreciation due to dirt on the luminaires and reduced lumen output from the in-service aging of lamps as factors that combine to decrease lighting system performance below design values. Maintained values in the range of 60 to 80 percent of initial design values are cited as common practice in this publication. With a particular focus on the needs of older drivers for increased illumination relative to younger motorists, to accommodate the age-related sensory deficits documented earlier in this discussion, a recommendation logically follows that lighting systems be maintained to provide service at the 80 percent level—i.e., the upper end of the practical range—with respect to their initial design values.

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P. Design Element: Pedestrian Control Devices

Table 20. Cross-references of related entries for pedestrian control devices.

A nationwide review of fatalities during the year 1985, and injuries during the period of 1983­1985, showed that 39 percent of all pedestrian fatalities and 9 percent of all pedestrian injuries involved the elderly (ages 64 and older) (Hauer, 1988). While the number of injuries is close to the population distribution (approximately 12 percent), the number of fatalities far exceeds the proportion of older pedestrians. The percentages of pedestrian fatalities and injuries occurring at intersections were 33 percent and 51 percent, respectively (Hauer, 1988). Accident types that predominantly involve older pedestrians at intersections are as follows (Blomberg and Edwards, 1990):

Age-related diminished capabilities, which may make it more difficult for older pedestrians to negotiate intersections, include decreased contrast sensitivity and visual acuity, reduced peripheral vision and useful field of view, decreased ability to judge safe gaps, slowed walking speed, and physical limitations resulting from arthritis and other health problems. Older pedestrian problem behaviors include a greater likelihood to delay before crossing, to spend more time at the curb, to take longer to cross the road, and to make more head movements before and during crossing (Wilson and Grayson, 1980).

Older and Grayson (1972) reported that although older pedestrians involved in accidents looked more often than the middle-aged group studied, over 70 percent of the adults struck by a vehicle reported not seeing it before impact. In a survey of older pedestrians (average age of 75) involved in accidents, 63 percent reported that they failed to see the vehicle that hit them, or to see it in time to take evasive action (Sheppard and Pattinson, 1986). Knoblauch, Nitzburg, Dewar, Templer, and Pietrucha (1995) noted that difficulty seeing a vehicle against a (complex) street background may occur with vehicles of certain colors, causing them to blend in with their background. This is especially problematic for older persons with reduced contrast sensitivity, who require a higher contrast for detection of the same targets than younger individuals, and who also have greater difficulty dividing attention between multiple sources and selectively attending to the most relevant targets. In addition, the loss of peripheral vision increases an older pedestrian's chances of not detecting approaching and turning vehicles from the side.

Reductions in visual acuity make it more difficult for older pedestrians to read the crossing signal. In a survey of older pedestrians in the Orlando, Florida, area, 25 percent of the participants reported difficulty seeing the crosswalk signal from the opposite side of the street (Bailey, Jones, Stout, Bailey, Kass, and Morgan, 1992).

Older pedestrians wait for longer gaps between vehicles before attempting to cross the road. In one study, approximately 85 percent of the pedestrians age 60 and older required a minimum gap of 9 s before crossing the road, while only 63 percent of all pedestrians required this minimum duration (Tobey, Shungman, and Knoblauch, 1983). The decline in depth perception may contribute to older persons' reduced ability to judge gaps in oncoming traffic. It may be concluded from these studies that older pedestrians do not process information (presence, speed, and distance of other vehicles) as efficiently as younger pedestrians, and therefore require more time to reach a decision. Other researchers have observed that older pedestrians do not plan their traffic behavior, are too trusting about traffic rules, fail to check for oncoming traffic before crossing at intersections, underestimate the speed of approaching vehicles, and follow other pedestrians without first checking for conflicts before crossing (Jonah and Engel, 1983; Mathey, 1983).

With increasing age, there is a concurrent loss of physical strength, joint flexibility, agility, balance, coordination and motor skills, and stamina. These losses contribute to slower walking speeds and difficulty negotiating curbs. In addition, older persons often fall as a result of undetected surface irregularities in the pavement and misestimation of curb heights. This results from a decline in contrast sensitivity and depth perception. In an assessment of 81 older residents (ages 70­97) to examine susceptibility to falling, 58 percent experienced a fall in the year following clinical assessment (Clark, Lord, and Webster, 1993). Impaired cognition, abnormal reaction to any push or pressure, history of palpitations, and abnormal stepping were each associated with falling. Knoblauch, Nitzburg, Reinfurt, Council, Zegeer, and Popkin (1995) reported that locating the curb accurately and placing the foot is a matter of some care, particularly for the elderly, the very young, and those with physical disabilities.

The studies discussed below define the types of accidents in which older pedestrians are most likely to be involved, and under what conditions the accidents most frequently occur. In addition, the specific geometric characteristics, traffic control devices (including signs, signals, and markings), and pedestrian signals that seem to contribute to older pedestrians' difficulties at intersections are discussed.

Zegeer and Zegeer (1988) stressed the importance of "tailoring" the most appropriate traffic control measures to suit the conditions at a given site. The effect of any traffic control measure is highly dependent on specific locational characteristics, such as traffic conditions (e.g., volumes, speeds, turning movements), pedestrian volumes and pedestrian mix (e.g., young children, college students, older adults, persons with physical disabilities), street width, existing traffic controls, area type (e.g., rural, urban, suburban), site distance, accident patterns, presence of enforcement, and numerous other factors.

Harrell (1990) used distance stood from the curb as a measure of pedestrian risk for intersection crossing. Observations of 696 pedestrians divided among 3 age groups (age 30 and under, ages 31­50, and age 51 and older) showed that the oldest group stood the farthest from the curb, that they stood even farther back under nighttime conditions, and that older females stood the farthest distance from the curb. The author used these data to dispel the findings in the literature that older pedestrians are not cognizant of the risks of exposure to injury from passing vehicles. Similarly, it may be argued that this behavior keeps them from detecting potential conflict vehicles and makes speed and distance judgments more difficult for them, while limiting their conspicuity to approaching drivers who might otherwise slow down if pedestrians were detected standing at the curbside at a crosswalk.

A study of pedestrian accidents conducted at 31 high-pedestrian accident sections in Maryland between 1974 and 1976 showed that pedestrians age 60 and older were involved in 53 (9.6 percent) of the accidents, and children younger than age 12 showed the same proportions. The pedestrians age 60 and older accounted for 25.6 percent of the fatal accidents. Compliance with traffic control devices was found to be poor for all pedestrians at all study locations; it was also found that most pedestrians keyed on the moving vehicle rather than on the traffic and pedestrian control devices. Only when the traffic volumes were so high that it was impossible to cross did pedestrians rely on traffic control devices (Bush, 1986).

Garber and Srinivasan (1991) conducted a study of 2,550 accidents involving pedestrians that occurred in the rural and urban areas of Virginia to identify intersection geometric characteristics and intersection traffic control devices that were predominant in crashes involving older pedestrians. Accident frequency by location and age for the accidents within the cities showed that while the highest percentage of accidents involving pedestrians age 59 and younger occurred within 46 m (150 ft) from the intersection stop line, the highest percentage of accidents for pedestrians age 60 years and older (51.8 percent) occurred within the intersection.

More recently, Knoblauch, Nitzburg, Reinfurt, et al. (1995) reported that, compared with younger pedestrians, older adults are overinvolved in crashes while crossing streets at intersections. In their earlier analysis of the national Fatal Accident Reporting System (FARS) data for the period 1980­1989, 32.2 and 35.3 percent of the deaths for pedestrians ages 65­74 and age 75 and older, respectively, occurred at intersections (Reinfurt, Council, Zegeer, and Popkin, 1992). This compared with 22 percent or less for the younger age groups. Analysis of the North Carolina motor vehicle crash file for 1980­1990 displayed somewhat smaller percentages, but showed the trend of increasing pedestrian accidents at intersections as age increased.

Further analysis of the North Carolina database showed that pedestrians age 65 and older as well as those ages 45­64 experienced 37 percent of their accidents on roadways with four or more lanes. This compares with 23.7 percent for pedestrians ages 10­44 and 13.6 percent for those age 9 and younger. The highest number of pedestrian-vehicle crashes occurred when the vehicle was going straight (59.7 percent), followed by a vehicle turning left (17.2 percent), and a vehicle turning right (13.3 percent). Right-turn crashes accounted for 18.9 percent of crashes with pedestrians ages 65­74, compared with 14.2 percent for pedestrians age 75 and older. The oldest pedestrian group was the most likely to be struck by a left-turning vehicle; they accounted for 23.9 percent of the crashes, compared with 18.1 percent of those ages 65­74 and 15.8 percent of those ages 45­64.

Knoblauch, Nitzburg, Reinfurt, et al. (1995) conducted a study to determine if pedestrian comprehension of and compliance with pedestrian signals could be improved by installing a placard that explained the three phases of pedestrian signals. They used findings from: (1) a focus group and workshop conducted in Baltimore, Maryland, with 13 participants ages 19­62 and (2) questionnaires administered to 225 individuals ages 19­80 and older at four Virginia Department of Motor Vehicles offices to determine the most effective message content and format for a pedestrian signal education placard. The newly developed placard was installed at six intersections in Virginia, Maryland, and New York. Observational studies of more than 4,300 pedestrians during 600 signal cycles found no change in pedestrian signal compliance. However, results from questionnaires administered to 92 subjects at Departments of Motor Vehicles in Virginia, Maryland, and New York indicated a significant increase in understanding of the phases of the pedestrian signal. The authors concluded that although pedestrian crossing behavior is more influenced by the presence or absence of traffic than the signal indication, the wording on the placard was based on quantitative procedures using a relatively large number of subjects and should be used where signal educational placards are installed. The wording of the educational placard recommended by Knoblauch, Nitzburg, Reinfurt, et al. (1995) is shown in Recommendation 2 of Design Element P. A modification for a two-stage crossing is shown in Recommendation 3.

Zegeer and Cynecki (1986) tested a LOOK FOR TURNING VEHICLES pavement marking in a crosswalk, as a low-cost countermeasure to remind pedestrians to be alert for turning vehicles, including right-turn-on-red (RTOR) vehicles. Results showed an overall reduction in conflicts and interactions for RTOR vehicles and also for the total number of turning vehicles. Even with an RTOR prohibition, approximately 20 percent of motorists committed an RTOR violation when given the opportunity (Zegeer and Cynecki, 1986). Of those violations, about 23.4 percent resulted in conflicts with pedestrians or vehicles on the side street.

Zegeer, Opiela, and Cynecki (1982) conducted an accident analysis to determine whether pedestrian accidents are significantly affected by the presence of pedestrian signals and by different signal timing strategies. They found no significant differences in pedestrian accidents between intersections that had standard-timed (concurrent walk) pedestrian signals compared with intersections that had no pedestrian signals. Concurrent or standard timing provides for pedestrians to walk concurrently (parallel) with traffic flow on the WALK signal. Vehicles are generally permitted to turn right (or left) on a green light while pedestrians are crossing on the WALK interval. Other timing strategies include early release timing, late release timing, and exclusive timing. In early release timing, the pedestrian WALK indication is given before the parallel traffic is given a green light, allowing pedestrians to get a head start into the crosswalk before vehicles are permitted to turn. In late release timing, the pedestrians are held until a portion of the parallel traffic has turned. Exclusive timing is a countermeasure where traffic signals are used to stop motor vehicle traffic in all directions simultaneously for a phase each cycle, while pedestrians are allowed to cross the street. "Barnes Dance" or "scramble" timing is a type of exclusive timing where pedestrians may also cross diagonally in addition to crossing the street. Exclusive timing is intended to virtually eliminate turning traffic or other movements that conflict with pedestrians while they cross the street. In the Zegeer et al. (1982) analysis, exclusive-timed locations were associated with a 50 percent decrease in pedestrian accidents for intersections with moderate to high pedestrian volumes when compared with both standard-timed intersections and intersections that had no pedestrian signals. However, this timing strategy causes excessive delays to both motorists and pedestrians. Older road users (age 65 and older) recommended the following pedestrian-related countermeasures for pedestrian signs and signals, during focus group sessions held as a part of the research conducted by Knoblauch, Nitzburg, Reinfurt, et al. (1995): (1) reevaluate the length of pedestrian walk signals due to increasingly wider highways, (2) implement more Barnes Dance signals at major intersections, and (3) provide more YIELD TO PEDESTRIANS signs in the vicinity of heavy pedestrian traffic.

The MUTCD (1988) indicates that a pedestrian clearance interval shall be provided when pedestrian signal indications are used, and should consist of a flashing DON'T WALK interval of sufficient duration to allow a pedestrian crossing in the crosswalk to leave the curb and travel to the center of the farthest traveled lane before opposing vehicles receive a green indication. The MUTCD (1988) assumes a normal walking speed of 1.22 m/s (4.0 ft/s). The Transportation and Traffic Engineering Handbook (Institute of Transportation Engineers [ITE], 1982), states that for relatively slow walkers, speeds of from 0.91 to 0.99 m/s (3.0 to 3.25 ft/s) would be more appropriate. Older pedestrian walking speed has been studied by numerous researchers. Sleight (1972) determined that there would be safety justification for use of speeds between 0.91 and 0.99 m/s (3.0 to 3.25 ft/s), based on the results of a study by Sjostedt (1967). In this study, average adults and the elderly had walking speeds of 1.37 m/s (4.5 ft/s); however, 20 percent of the older pedestrians crossed at speeds slower than 1.22 m/s (4.0 ft/s). The 85th percentile older pedestrian walking speed in that study was 1.04 m/s (3.4 ft/s). A 1982 study by the Minnesota Department of Transportation found that the average walking speed of older pedestrians was 0.91 m/s ( 3.0 ft/s). In a study conducted in Florida, it was found that a walking speed of 0.76 m/s (2.5 ft/s) would accommodate 87 percent of the older pedestrians observed (ITE, undated). Weiner (1968) found an average rate for all individuals of 1.29 m/s (4.22 ft/s), and of 1.13 m/s (3.7 ft/s) for women only. A Swedish study by Dahlstedt (undated), using pedestrians age 70 and older, found that the 85th percentile comfortable crossing speed was 0.67 m/s (2.2 ft/s).

Hoxie and Rubenstein (1994) measured the crossing times of older and younger pedestrians at a 21.85-m- (71.69-ft-) wide intersection in Los Angeles, CA, and found that older pedestrians (age 65 and older) took significantly longer than younger pedestrians to cross the street. In this study, the average walking speed of the older pedestrians was 0.86 m/s (2.8 ft/s), with a standard deviation of 0.17 m/s (0.56 ft/s); the average speed of the younger pedestrians was 1.27 m/s (4.2 ft/s), with a standard deviation of 0.17 m/s (0.56 ft/s). Of the 592 older pedestrians observed, 27 percent were unable to reach the curb before the light changed to allow cross traffic to enter the intersection, and one-fourth of this group were stranded at least a full traffic lane away from safety.

More recently, Knoblauch, Nitzburg, Dewar, et al. (1995) conducted a series of field studies to quantify the walking speed, start-up time, and stride length of pedestrians younger than age 65 and pedestrians 65 and older under varying environmental conditions. Analysis of the walking speeds of 3,458 pedestrians younger than age 65 and 3,665 pedestrians age 65 and older crossing at intersections showed that the mean walking speed for younger pedestrians was 1.51 m/s (4.95 ft/s) and for older pedestrians was 1.25 m/s (4.11 ft/s). The 15th percentile speeds were 1.25 m/s and 0.97 m/s (4.09 ft/s and 3.19 ft/s) for younger and older pedestrians, respectively. These differences were statistically significant. Among the many additional findings with regard to walking speed were the following: pedestrians who start on the WALK signal walk slower than those who cross on either the flashing DON'T WALK or steady DON'T WALK; the slowest walking speeds were found on local streets while the faster walking speeds were found on collector-distributors; sites with symbolic pedestrian signals had slower speeds than sites with word messages; pedestrians walk faster where RTOR is not permitted, where there is a median, and where there are curb cuts; faster crossing speeds were found at sites with moderate traffic volumes than at sites with low or high vehicle volumes.

For design purposes, a separate analysis was conducted by Knoblauch, Nitzburg, Dewar, et al. (1995) for pedestrians who complied with the signal, as they tended to walk more slowly than those who crossed illegally. The mean crossing speed for the young compliers was 1.46 m/s (4.79 ft/s) and for the older compliers was 1.20 m/s (3.94 ft/s). The 15th percentile speed for the young compliers was 1.21 m/s (3.97 ft/s) and was 0.94 m/s (3.08 ft/s) for the older compliers. Older female compliers showed the slowest walking speeds, with a mean speed of 1.14 m/s (3.74 ft/s) and a 15th percentile of 0.91 m/s (2.97 ft/s). One of the slowest 15th percentile values (0.89 m/s [2.94 ft/s]) was observed for older pedestrians crossing snow-covered roadways. It was concluded from this research that a mean design speed of 1.22 m/s (4.0 ft/s) is appropriate, and where a 15th percentile is appropriate, a walking speed of 0.91 m/s (3.0 ft/s) is reasonable. It was also determined by Knoblauch, Nitzburg, Dewar, et al. (1995) that the slower walking speed of older pedestrians is due largely to their shorter stride lengths. The stride lengths of all older pedestrians are approximately 86 percent of those of younger pedestrians.

Finally, Knoblauch, Nitzburg, Dewar, et al. (1995) also measured start-up times for younger and older pedestrians who stopped at the curb and waited for the signal to change before starting to cross. The mean value for younger pedestrians was 1.93 s compared with 2.48 s for older pedestrians. The 85th percentile value of 3.06 s was obtained for younger pedestrians, compared with 3.76 s for older pedestrians. For design purposes, the authors concluded that a mean value of 2.5 s and an 85th percentile value of 3.75 s would be appropriate. These data specifically did not include pedestrians using a tripod cane, a walker, or two canes; people in wheelchairs; or people walking bikes or dogs. The MUTCD (1988) states that under normal conditions, the WALK interval should be at least 4 to 7 s in length so that pedestrians will have adequate opportunity to leave the curb before the clearance interval is shown. Parsonson (1992) noted that the reason this much time is needed is because many pedestrians waiting at the curb watch the traffic, and not the signals. When they see conflicting traffic coming to a stop, they will then look at the signal to check that it has changed in their favor. If they are waiting at a right-hand curb, they will often take time to glance to their left rear to see if an entering vehicle is about to make a right turn across their path. Parsonson reported that a pedestrian reasonably close to the curb and alert to a normal degree can be observed to require up to 4 or 5 s for this reaction, timed from when the signal changes to indicate that it is safe to cross, to stepping off the curb. It may be remembered that older pedestrians stand farther away from the curb, and may or may not be alert. In addition, there are many drivers who run the amber and red signals, and it is prudent for pedestrians to "double-check" that traffic has indeed obeyed the traffic signal, and that there are no vehicles turning right on red or (permissive) left on green before proceeding into the crosswalk. Because older persons have difficulty dividing attention, this scanning and decisionmaking process requires more time than it would for a younger pedestrian. Parsonson (1992) reported that the State of Delaware has found that pedestrians do not react well to the short WALK and long flashing DON'T WALK timing pattern. They equate the flashing with a vehicle yellow period. The Florida Department of Transportation and the city of Durham, Ontario, provide sufficient WALK time for the pedestrian to reach the middle of the street, so that the pedestrian will not turn around when the flashing DON'T WALK begins.

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