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Older Driver Highway Design Handbook

III. ROADWAY CURVATURE AND PASSING ZONES

1. Pavement Markings and Delineation on Horizontal Curves
2. Pavement Width on Horizontal Curves
3. Crest Vertical Curve Length and Advance Signing for Sight-Restricted Locations
4. Passing Zone Length, Passing Sight Distance, and Passing/Overtaking Lanes on Two-Lane Highways
   

A. Design Element: Pavement Markings and Delineation on Horizontal Curves

Table 26. Cross-references of related entries for pavement markings and delineation on horizontal curves.

Note: Page letter references (e.g., 3A-2) refer to the MUTCD (1988), while those with only numbers (e.g., Pg. 71) refer to Rev Part VI of the MUTCD (1993).

Pavement markings and delineation devices serve important path guidance functions on horizontal curves, particularly under adverse visibility conditions, at twilight, and at nighttime. They provide a preview of roadway features ahead and give the driver information about the vehicle's lateral position on the roadway. Delineation must provide information that results in recognition of the boundaries of the traveled way both at "long" preview distances (5 to 8 s of travel time) and at more immediate proximities (within 1 s of travel time) where attention is directed toward instant-to-instant vehicle control responses.

Surface pavement markings in current practice may vary along four dimensions: brightness, width, thickness, and the addition of structure to "thick" applications. Stripes of increased thickness have an advantage in wet weather because the material is more likely to protrude above the level of surface water and to provide a degree of retroreflectivity greater than that provided by thinner applications of paint. Also, the commercially available structured stripes (tapes) are brighter than other marking treatments, even under dry conditions. This is due to the ability of the raised element of the structure to reflect more light back to the driver than a horizontal surface. Even greater benefits are provided by reflectorized treatments, including raised pavement markers (RPM's), post-mounted delineators (PMD's), and chevron signs, which may be used to improve the nighttime visibility of delineation and to indicate roadway alignment.

A number of driver visual functions that have an impact on the use of pavement markings and delineation show significant age-related decrements: dynamic acuity, contrast sensitivity, dark adaptation, and glare recovery. Dynamic visual acuity (DVA) includes the ability to resolve the details of a high-contrast target that is moving relative to an observer. Activities that rely on dynamic acuity include making lateral lane changes and locating road boundaries when negotiating a turn. In these situations, greater speeds are associated with poorer DVA. Contrast sensitivity influences the response to both sharply defined, bright versus dark visual targets, and those with grayer, less distinct edges. In general, older adults tend to have decreased contrast sensitivity (Owsley, Sekuler, and Siemsen, 1983). This loss is more pronounced at lower light levels (Sloane, Owsley, and Alvarez, 1988; Sloane, Owsley, and Jackson, 1988) and is associated with a heightened sensitivity to glare (Wolf, 1960; Fisher and Christie, 1965; Pulling, Wolf, Sturgis, Vaillancourt, and Dolliver, 1980). The findings of Blackwell and Blackwell (1971) indicate that a 60-year-old observer needs approximately 2.5 times the contrast as a 23-year-old observer for the same level of visibility.

Highway research studies that have varied one or more of the four dimensions of pavement markings are discussed below, along with studies on the effectiveness of RPM's, PMD's, chevron signs, and combinations of delineation treatments. Age differences are reported wherever data are available.

An early study of surface pavement markings employing an interactive driving simulator, plus field evaluations, 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 (Blackwell and Taylor, 1969). 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, these studies have asserted that markings must appear to be at least three times as bright as the road surface, because contrast is defined as the difference between target and background luminance, divided by the background luminance alone. A difficulty with these studies, however, is that their data were not derived from—and thus are not representative of—normatively aged older drivers. The ideal viewing conditions assumed by Allen et al. (1977) also disregard the effects of glare as well as adverse visibility, and both factors have a disproportionate impact on the performance of older drivers. In Blackwell and Taylor's work, a minimum preview time of 3 to 4 s was recommended for accurate maneuvering under adverse conditions. However, more conservative estimates of preview time to accommodate older drivers (e.g., 5 s) have frequently appeared in the literature.

Freedman, Staplin, Gilfillan, and Byrnes (1988) showed significant performance decrements for 65-year-old drivers, as compared with 35-year-old drivers, in the visibility distance of 100-mm (4-in) pavement stripes on a simulated wet roadway. Staplin, Lococo, and Sim (1990) confirmed the need for higher levels of line brightness for older drivers in a simulator study, where the contrast for a 100-mm (4-in) white edgeline was continuously varied within a 40-step range in a method of limits. Under simulated opposing headlamp glare conditions, subjects ages 65­80 required an increase in contrast of 20 to 30 percent over a younger sample to correctly discern downstream curve direction at criterion viewing distances. To accommodate less capable older drivers, this study's results indicated that an increase in stripe brightness that is tenfold greater (300 percent) for older versus younger drivers may be warranted.

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 earlier in this Handbook in table 9 for Design Element F (Edge Treatments/Delineation of Curbs, Medians, and Obstacles) in the Rationale and Supporting Evidence section for Intersections (At-Grade). 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 (most capable) of 25-year-olds and bottom 5 percent (least capable) of 75-year-olds were compared in this analysis.

The more realistic operating conditions modeled as described above, together with the widely cited multiplier for older observers advocated in the seminal work by Blackwell and Blackwell (1971), support the recommendation that an in-service pavement edge striping contrast value on horizontal curves maintained at or above 5.0 is appropriate to accommodate the needs of the large majority of older drivers on highways and arterials without median separation between opposing directions of traffic. Where a median barrier (e.g., Jersey barrier) high enough to shield drivers from direct view of oncoming headlights is present, or where median width exceeds 15 m (49 ft), a horizontal curve edgeline contrast value of 3.75 or higher is recommended. It is important to note that these recommendations assume the standard stripe width of 100 mm (4 in). Where wider pavement markings are implemented, either as general or spot treatments, the same contrast values apply. It may be inferred from various studies of stripe width (e.g., Good and Baxter, 1986; Deacon, 1988) that treatments that are maintained at or above the recommended contrast levels and are wider than 100 mm (4 in) will provide the greatest benefit to older drivers. Contrast remains the preeminent factor in stripe visibility, however, and increased width alone does not substitute for lower-than-recommended contrast levels.

Raised pavement markers have received widespread use because they provide better long-range delineation than conventional painted lines, particularly under wet conditions. When used on a road edge, they also provide brighter peripheral cues, which could be advantageous to the older driver for path guidance. Over time, however, RPM's also are subject to loss of their initial retroreflectivity; in colder climates, RPM's may be damaged by plowing operations.

Deacon (1988), in his review of research on delineation and marking treatments that he believed would be of particular benefit to the older driver, found that highways with RPM centerlines had lower crash rates than those with painted centerlines. The average reduction in crash rates was approximately 0.5 crashes per million vehicle-miles. Zador, Stein, Wright, and Hall (1986) observed that after-modification vehicle paths were shifted away from the centerline on right and left curves with RPM's mounted on both sides of the double yellow centerlines, and that placement changes were largest with RPM's compared with PMD's and chevrons. It has also been observed that RPM's placed in the centerlines and edgelines at pavement width reductions at narrow bridges produce significant reductions in 85th percentile speeds and centerline encroachments (Niessner, 1984). On two-lane rural curves, RPM's in conjunction with the double yellow centerline have been recommended.

An RPM spacing study was conducted by Blaauw (1985), who tested several RPM patterns on 200-m (656-ft) radii and 1,000-m (3,281-ft) radii horizontal curves using a visual occlusion technique. White RPM's were used for the tests, at spacing distances of approximately 12.2 m, 24.4 m, and 36.6 m (40 ft, 80 ft, and 120 ft). On 200-m (656-ft) radius curves, the 24.4-m and 36.6-m (80-ft and 120-ft) spacings led to speed reductions and lane errors. Based on these results, it was recommended that on curves of this severity, the spacing of RPM's be restricted to 12.2-m (40-ft) spacings. In general, no differences between treatments were observed for the more gentle, 1,000-m (3,281-ft) radius curves.

Accordingly, this Handbook includes a recommendation for RPM installation, at standard (12.2-m [40-ft]) spacings, on all horizontal curves with radii below 1,000 m (3,281 ft).

Roadside delineators and treatment combinations are also important to this discussion. Because of its increasing use throughout the United States, and because it accommodates different types of sheeting in varying amounts and different designs, the primary roadside delineation device of current interest is the flat, flexible post. The general accident data have shown that the installation of PMD's is associated with lower crash rates for highway sections with or without edgelines (Bali, Potts, Fee, Taylor, and Glennon, 1978; Schwab and Capelle, 1979). Deacon (1988) confirmed that installation of PMD's lowered crash rates, for sections with or without edgelines. The reduction in crash rates resulting from the installation of these delineators averaged 1.0 crashes per million vehicle-miles. Thus, especially for lower functional classification roadways where the use of enhanced (e.g., wider) edgelines may be limited (due to pavement width restrictions), existing data suggest that PMD's can be an effective countermeasure.

In a driver performance study evaluating the effects of chevron signs, PMD's, and RPM's, both Johnson (1984) and Jennings (1984) found that driver performance on sharp curves was the most favorable when chevrons were used. With chevrons, drivers followed a better path around the curve (defined in terms of the ratio of the vehicle's instantaneous radius to the actual curve radius). These studies also revealed that drivers use a corner-cutting strategy, and that chevron signs and PMD's facilitated this strategy. On right curves with chevrons, drivers had an average midcurve placement closest to the centerline. On left curves with chevrons, vehicle placement was not significantly different. In the Good and Baxter (1986) study, chevron signs had a detrimental effect on control behavior, but were rated favorably by drivers in reducing task difficulty. Zador et al. (1986) found that chevrons (as well as RPM's) tend to shift vehicles away from the centerline on right and left curves, while PMD's shift vehicles away from the centerline on right curves. A particular advantage for chevrons with high-intensity reflective sheeting was demonstrated for drivers age 65 and older in a study by Pietrucha, Hostetter, Staplin, and Obermeyer (1994), when used in combination with other treatments.

The Pietrucha et al. (1994) study was specifically directed to the difficulties older drivers have with horizontal curve delineation elements, and the possible benefits of brighter materials, larger target sizes, redundant and/or multidimensional cues using combinations of elements, and novel designs or configurations of elements. Twenty-five distinct delineation/pavement marking treatments (a baseline treatment and 24 enhancements) were initially presented to subjects in 3 driver age groups (18­45, 65­74, and 75 and older). The baseline treatment was a 100-mm
(4-in) yellow centerline at in-service brightness level (ISBL). The 24 treatments varied according to the presence/absence of edgeline, edgeline width, whether the edgeline was enhanced with RPM's, whether the centerline was enhanced with RPM's, and the presence/absence of off-road elements and their characteristics (material, color, brightness, and/or spacing). Measures of effectiveness were downstream roadway feature recognition (subjects were required to report the direction in which the roadway curved) and recognition distance in a 35-mm simulation of nighttime driving. Treatments that included the addition of RPM's to both the centerline and edgeline, and all treatments that included delineating the roadway edge with high-intensity chevrons or high-intensity PMD's, resulted in significantly higher mean recognition distances when compared with the baseline treatment, across all age groups. For the subjects age 65 and older, only a subset of the treatments with delineated roadway edges resulted in significantly higher mean recognition distances, due to the increased variance among older subjects' data. Next, field evaluations were conducted with a subset of the most promising treatments. The treatment with the highest recognition distance for both age groups consisted of a 100-mm (4-in) yellow centerline at ISBL with yellow RPM's at ISBL and standard spacing, a 100-mm- (4-in)- wide white edgeline, and fully reflectorized T-post delineators with standard spacing. For the 152.4-m (500-ft) radius of curvature used in this study, spacing for the PMD's was 19.8 m (65 ft). This treatment included PMD's that were fully reflectorized, i.e., retroreflective material extended from the top of the post to the ground and provided more reflective area than the standard posts most frequently used.

Blaauw (1985) tested combinations of PMD's and RPM's, resulting in the following recommendations: (1) RPM's exclusively at the center are favorable for lateral vehicle control inside the lane (short-range delineation) but are less adequate for preview information on the lane to be followed (long-range delineation); therefore, it is necessary to delineate both lane boundaries; (2) effective centerline delineation can be realized with RPM's; (3) delineation at the outside of the traffic lane can be realized with RPM's at the location of the lane boundary or with PMD's spaced laterally at 1.5 m (5 ft)—both configurations are equally efficient, but PMD's at an approximate 3.7-m (12-ft) spacing are less efficient; and (4) RPM's at the location of the center and/or lane boundaries must be applied with a maximum spacing distance of 12 m (40 ft) on a curve with 200-m (656-ft) radius or less.

In a laboratory study of drivers' responses to videotapes of four rural horizontal curves, six levels of delineation plus two levels of curvature were studied by Rockwell and Smith (1985). The levels included no delineation; centerline only; centerline plus edgeline; centerline plus edgeline plus PMD's; centerline plus edgeline plus RPM's; and centerline plus edgeline plus PMD's plus RPM's. Subjects were required to identify precisely the instant that they could detect the presence of a curve and then express their level of confidence with their response. The largest increase in detection distance was associated with the addition of RPM's and PMD's to the centerline and edgeline treatments, respectively.

While no specific roadside treatment on horizontal curves is advocated in this Handbook, a recommendation for roadside delineation devices at minimum spacings keyed to curve radius appears justified by the findings reported above. Using current practice as a guide, a spacing of 12 m (40 ft) represents an average value in table III-1 of the MUTCD, Suggested Spacing for Highway Delineators on Horizontal Curves, for curves with radii from 15 to 150 m (50 to 500 ft). This value is also consistent with the 12-m (40-ft) spacing requirement for RPM's on curves with radii 200 m (656 ft) noted above.

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B. Design Element: Pavement Width on Horizontal Curves

Roadway alignment is a key factor in unsafe vehicular operation: i.e., increasing degrees of curvature cause more accidents (Haywood, 1980). The widening of lanes through horizontal curves, minimizing the use of controlling or maximum curvature for a given design speed, and the use of special transition curves for higher speed and sharper curve designs have all been suggested as countermeasures. Whereas in the past lane widening has been advocated to accommodate the tracking of large trucks through curves, the present focus is on the accommodation of older drivers, whose diminished physical and perceptual abilities make curve negotiation more difficult. Lane widths on horizontal curves range from 2.7 m to 4 m (9 ft to 13 ft), but are usually 3.4 m or 3.7 m (11 ft or 12 ft) wide. Neuman (1992) recommended that when less than 3.7-m- (12-ft)-wide lanes are used, consideration should be given to widening the lane to this dimension through horizontal curves; and a further increase in width of 0.3­0.6 m (1­2 ft) may be advised to provide for an additional margin of safety through the curve for heavy vehicles. This margin of safety could also be justified in terms of its benefit to older drivers with diminished physical abilities.

Older drivers, as a result of age-related declines in motor ability, have been found to be deficient in coordinations involved in lanekeeping, maintaining speed, and handling curves (Brainin, Bloom, Breedlove, and Edwards, 1977). McKnight and Stewart (1990) also reported that older drivers have difficulty in lanekeeping, which results in frequently exceeding lane boundaries, particularly on curves. Drivers who lack the required strength, including older drivers and physically limited drivers, often swing too wide in order to lengthen the turning radius and minimize rotation of the steering wheel.

Joint flexibility is an essential component of driving skill. Osteoarthritis, the most common musculoskeletal disability among older individuals, affects more than 50 percent of the population age 65 and older (Roberts and Roberts, 1993). If upper extremity range of movement is impaired in the older driver, mobility and coordination are seriously weakened. Older drivers with some upper extremity dysfunction may not be able to steer effectively with both hands gripping the steering wheel rim. In a study of 83 people with arthritis, 7 percent used the right hand only to steer and 10 percent used only the left hand (Cornwell, 1987).

The general relationship between pavement width and safe driving operations has been well documented. Choueiri and Lamm (1987) reported the results of several early studies that found an association between decreasing accident frequency and increasing pavement widths. Krebs and Kloeckner (1977) reported that for every 1-m (3.3-ft) increase in pavement width, a decrease of 0.25 in the accident rate (per million vehicle-kilometers) could be expected. Hall, Burton, Coppage and Dickinson (1976) examined the nature of single-vehicle accidents involving fixed objects along the roadside of nonfreeway facilities. They found that the majority of these types of accidents were reported as nonintersection related, and occurred most frequently on weekends, at night, under adverse pavement and weather conditions, and on horizontal curves (especially outside of curve). These accident types have high injury severity to drivers and passengers. Wright and Robertson (1979) reported that 40 and 31 percent of all fatal crashes in Pennsylvania and Maryland, respectively, resulted in a vehicle hitting a fixed object such as a tree, utility pole, or bridge abutment. In a study focused on 600 accident sites (and 600 comparison sites) involving fixed objects, crash locations were best discriminated from comparison locations by a combination of curvature greater than 9 degrees and downhill gradient steeper than 3 percent; and, for the fatal fixed-object crash population, the crash locations were best discriminated from comparison locations by a combination of curvature greater than 6 degrees and downhill gradient steeper than 2 percent.

Glennon and Weaver (1971) evaluated the adequacy of geometric design standards for highway curves by filming vehicles entering unspiraled highway curves with curvature ranging from 2 to 7 degrees. While driver age was not analyzed, results of the study indicated that most vehicle paths, regardless of speed, exceed the degree of highway curve at some point on the curve. Glennon, Neuman, and Leisch (1985) measured vehicle speed and lateral placement on horizontal curves and found that drivers tend to overshoot the curve radius, producing minimum vehicle path radii sharper than the highway curve, and that the tendency to overshoot is independent of speed. They observed that the tangent alignment immediately in advance of the curve is the critical region of operations, because at about 61 m (200 ft) before the beginning points of the curve (or approximately 3 s driving time), drivers begin to adjust both their speed and path. Such adjustments are particularly large on sharper curves. Thus, the margin of safety in current AASHTO design policy is much lower than anticipated.

Zegeer, Stewart, Reinfurt, Council, Neuman, Hamilton, Miller, and Hunter (1990) conducted a study to determine the horizontal curve features that affect accident experience on two-lane rural roads and to evaluate geometric improvements for safety upgrading. An analysis of 104 fatal and 104 nonfatal accidents on rural curves in North Carolina showed that in more of the fatal accidents, the first maneuver was toward the outside of the curve (77 percent of the fatal accidents versus 64 percent of the nonfatal accidents). For approximately 28 percent of the fatal accidents (versus 8.8 percent of the nonfatal accidents), the vehicle ran off the road to the right and then returned to be involved in a crash. Further, an analysis on 10,900 horizontal curves in the State of Washington with corresponding accident, geometric, traffic, and roadway data variables showed that the percentages of severe nonfatal injuries and fatalities were greater on curves than on tangents with the same width, where total road width (lanes plus shoulders) was 9 m (30 ft).

Zegeer et al. (1990) concluded that widening lanes or shoulders on curves can reduce curve accidents by as much as 33 percent. Specifically, table 28 shows the predicted percent reduction in accidents that would be expected on horizontal curves by widening the lanes and by widening paved and unpaved shoulders (Zegeer et al., 1990).

Table 28. Percent reduction in accidents on horizontal curves with 2.4 m (8 ft) beginning lane width as a result of lane widening, paved shoulder widening, and unpaved shoulder widening. Source: Zegeer et al., 1990.

1 ft = 0.305 m

* Values of lane widening correspond to a maximum widening of 8 ft (2.4 m) to 12 ft (3.7 m) for a total of 4 ft (1.2 m) per lane, or a total of 8 ft (2.4 m) of widening.

The evidence cited above from the engineering studies describing curve negotiation, pavement width, and accident reduction, together with the documented difficulties in lanekeeping and diminished motor abilities of older drivers, support the recommendation for a minimum pavement width (including shoulder) of 5.5 m (18 ft) on arterial horizontal curves over 3 degrees of curvature (cf. Cirillo and Council, 1986). It is understood that limited-access highways already exceed this recommended lane-plus-shoulder width. However, older drivers often report a preference to travel on two-lane arterials, and these facilities may be deficient in this regard, especially in rural settings.

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C.Design Element: Crest Vertical Curve Length and Advance Signing for Sight-Restricted Locations

Table 29. Cross-references of related entries for crest vertical
curve length and advance signing for sight-restricted locations.

Note:Page letter references (e.g., 2C-22) refer to the MUTCD (1988), while those with only numbers (e.g., Pg. 40) refer to Rev Part VI of the MUTCD (1993).

From a human factors perspective, the accommodation of older driver needs should be a high priority at sight-restricted locations because of the potential for violation of expectancy, even though the actual percentage of accidents occurring under conditions of limited (vertical) sight distance is quite small (Pline, 1996). Older drivers, as a result of their length of experience, develop strong expectations about where and when they will encounter roadway hazards and "high-demand" situations with increased potential for conflict. At the same time, older driver reaction time is slower in response to unexpected information, and older drivers are slower to override an initial incorrect response with the correct response. Further, aging is associated with physical changes that may interfere with rapid vehicle control when an emergency maneuver is required.

Of greatest importance during the approach to sight-restricted locations are the cognitive components of driving, most notably selective attention and response speed (complex reaction time). Selective attention refers to the ability to identify and allocate attention appropriately to the most relevant targets at any given time (Plude and Hoyer, 1985). One important finding in the selective attention literature, as noted above, is that older adults respond much more slowly to stimuli that are unexpected (Hoyer and Familant, 1987), suggesting that older adults might be particularly disadvantaged when an unexpected hazard appears in the road ahead. In fact, Stansifer and Castellan (1977) suggested that hazard recognition errors can be interpreted more as attention failures than as sensory deficiencies. The selective attention literature suggests that for adults of all ages, but perhaps particularly for the elderly, the most relevant information should be signaled in a dramatic manner to ensure that it receives a high priority for processing.

Next, appropriate vehicle control behaviors when unexpected hazards are encountered depend upon "speeded responding," or how quickly an individual is able to respond to a relevant target, once identified. A timely braking response when one recognizes that the car ahead is stopped or that a red signal or STOP sign is present can determine whether or not there is a crash. Thus, reaction time or the ability to respond quickly to a stimulus is a critical aspect of successful driving. Mihal and Barrett (1976) measured simple, choice, and complex reaction time and reported that simple and choice reaction time were not correlated with accidents, but complex reaction time was. Moreover, when only older adults were examined, the correlation with accident involvement increased from 0.27 for complex reaction for the total sample to 0.52, suggesting the relationship to be particularly marked for older adults. There is nearly uniform agreement among researchers that reaction time (speed) decreases with age. In particular, studies have demonstrated a significant and disproportionate slowing of response for older adults versus young and middle-aged adults as uncertainty level increased for response preparation tasks. Preparatory intervals and length of precue viewing times appear to be crucial determinants of age-related differences in movement preparation and planning (Eisdorfer, 1975; Stelmach, Goggin, and Garcia-Colera, 1987; Goggin, Stelmach, and Amrhein, 1989).

In summary, the age-related deficits in reaction time and various aspects of attention are not independent of one another, and more than one of these mechanisms is likely to reduce driving efficiency in the older adult. Because of these deficits, sight-restricted locations pose a particular risk to older drivers, presenting a need for recommendations addressing both geometry and signing that can be reconciled with available highway research findings in this area.

Unfortunately, there is a lack of conclusive data on this subject. Kostyniuk and Cleveland (1986) analyzed the accident histories of 10 matched pairs of sites on 2-lane rural roadways. The 10 limited sight distance (vertical curve) locations were defined as those below the minimum stopping sight distance (SSD) recommended by AASHTO in 1965, and ranged from 36 m to 94 m (118 ft to 308 ft). The control site locations were defined as those that more than met the standard (SSD greater than 213 m [700 ft]). Seven of the limited sight distance sites had more accidents than the matched control sites, two were approximately equal, and one had fewer accidents (Pline, 1996). Overall, the set of sites with less than minimum SSD had over 50 percent more accidents in the study period than the control sites.

Farber (1982) performed sensitivity analyses of the effects of change in eye height, object height, friction, and speed on SSD on crest vertical curves. He found that SSD was relatively insensitive to a reasonable range of changes in driver eye height, but was very sensitive to speed, friction, and reaction time. Thus, stopping distance on vertical curves that are of inadequate length or are substandard according to other design criteria, and where major redesign, repaving, or excavation is not feasible, could most efficiently be made safer by modifying a driver's approach speed and/or reaction time. For 88 km/h (55 mi/h) traffic, stopping distance increases 24.7 m (81 ft) for every 1-s increase in reaction time. Similarly, stopping distance decreases about 4.9 m for each 1-km/h reduction in speed (or 26 ft for each 1 mi/h). A need for more effective traffic control countermeasures is thus highlighted.

A reevaluation of crest vertical curve length requirements was performed by Khasnabis and Tad (1983). These researchers reviewed the historical changes in parameters that affect the computation of SSD and evaluated the effect of these changes on the length requirements of crest vertical curves. Principal conclusions were that further tests on reaction time are needed, since the current 2.5-s reaction time may not accurately reflect the changing age distribution and composition of the driving population. In addition, the validity of the assumption of a speed differential for wet pavement conditions between design speed and top driving speed is questionable, since there is very little evidence to substantiate the assumption that all motorists are likely to reduce their speed on wet pavements. Of particular interest, Khasnabis and Tad (1983) noted that the object height of 150 mm (6 in) appears to be somewhat arbitrary (i.e., the current AASHTO design criterion), and stated that reducing the object height to 75 mm (3 in) would improve the safety elements of crest curves.

In contrast, there are strong proponents of the position that the obstacle height criterion for design of vertical curves should be raised to 450 mm (18 in), or the approximate height of a passenger vehicle's rear taillights (see Neuman, 1989). While McGee (1995) has reported that available data are insufficient to definitively establish the relationship between (limitations in) vertical alignment and highway safety, and there is an indisputable logic in using a height criterion corresponding to the most commonly encountered obstacle on the road (i.e., another vehicle), this approach disproportionately penalizes older drivers in those rare circumstances when a hazard (of any type) appears unexpectedly due to sight-restricting geometry. Also, the simple argument that a conclusive relationship cannot be demonstrated as justification for changing current practice is somewhat disingenuous—a significant relationship between visual acuity and accident involvement has proven elusive, over decades of study, yet there is widespread acknowledgment that good vision is necessary for safe driving.

Returning to a consideration of potential countermeasures, as stopping distance is sensitive to decreases in speed and reaction time, any traffic control device that lowers either parameter is beneficial. In one study, a LIMITED SIGHT DISTANCE (W14-4) sign with a speed advisory was found to be understood by only 17 percent of the 631 respondents who passed through the study sight (Christian, Barnack, and Karoly, 1981). Part of the problem may be that unlike the hazards cited by other warning signs, the phrase "limited sight distance" has no tangible manifestation, and even when drivers have topped the crest of a vertical curve, they may not be aware of the extent to which their sight distance was reduced. Freedman, Staplin, Decina, and Farber (1984) developed and tested the effectiveness of both verbal and symbol alternative warning devices for use on crest vertical curves using drivers ages 16­75. The existing LIMITED SIGHT DISTANCE sign, with or without a supplementary speed advisory panel, did not produce desirable driver responses (braking or slowing) as frequently, nor was it recalled, comprehended, recognized, or preferred as often as the verbal alternative SLOW HILL BLOCKS VIEW sign, or an alternative symbol sign that depicted two vehicles approaching from opposite sides of a hill.

With a focus on the conspicuity and legibility of static warning signs (i.e., as may be placed in advance of sight-restricted locations), a survey by the American Automobile Association Foundation for Traffic Safety found that 25 percent of older drivers experienced problems reading traffic signs (Yee, 1985). Olson and Bernstein (1979) suggested that older drivers should not be expected to achieve an legibility index (LI) of 0.6 m/mm (50 ft/in) under most nighttime circumstances. The data provided by this report give some expectation that 0.48 m/mm (40 ft/in) is a reasonable goal under most conditions for an "average" driver, that is one whose performance is at the 50th percentile (median) level for his or her age. To accommodate less capable older drivers, an LI of less than 0.48 m/mm (40 ft/in) would be of clear benefit. Larger sign panel sizes may be required to accommodate the larger characters necessary to achieve this LI for some messages.

Several studies have shown that the use of active sign elements, such as flashing warning lights for SLOW WHEN FLASHING and MAX SPEED ___MPH messages supplementing various standard warning signs, increases the conspicuity of the signs and results in greater speed reductions (Zegeer, 1975; Hanscomb, 1976; Lanman, Lum, and Lyles, 1979; Lyles, 1981) as well as a 60­70 percent reduction of accidents at grade crossings compared with the static sign alone conditions (Hopkins and Holmstrom, 1976; Hopkins and White, 1977). According to Pline (1996), several agencies have experienced success with the use of flasher-augmented warning signs with the legend PREPARE TO STOP when there is limited sight distance to a signalized intersection, activated at the time of signal change (red phase).

Lyles (1980) compared the effects of warning signs at horizontal and crest vertical curves with limited sight distance (less than 152.4 m [500 ft]). Five warning devices were evaluated: (1) the standard intersection crossroad warning symbol sign; (2) a warning sign with the message VEHICLES ENTERING; (3) a sequence of two warning signs and a regulatory sign (REDUCED SPEED AHEAD, crossroad symbol, and 35 mph speed limit sign); (4) a VEHICLES ENTERING sign with constantly flashing warning lights; and (5) the same as (4) but with a WHEN FLASHING plate, with flashing warning lights activated only in the presence of crossroad traffic. Overall, the standard crossroads and VEHICLES ENTERING signs had less speed-reducing effect (0.8­3.2 km/h [0.5­2 mi/h]) than the warning-warning-regulatory sequence and the signs with warning lights (6.4­8 km/h [4­5 mi/h]). This trend was the same for both horizontal and vertical curves, and there was no significant difference between the warning-warning-regulatory sequence and the signs with warning lights. Motorists were twice as likely to recall the warning-warning-reg-
ulatory sequence and signs with warning lights than the standard signs, and a van positioned at the crossroad was also reported to have been seen more often with these sign configurations.

As reviewed above, studies have shown that, in general, approach speeds to crest vertical curves make safe response by older drivers to a revealed obstacle unlikely given current design criteria. There is ample evidence of significant age-related declines in response capability to unexpected hazards. Analyses of curve length requirements conclude that safety benefits will result from a lower object height, yet practical considerations have prompted a move toward a higher criterion. Retention of the 150-mm (6-in) criterion is the most prudent practice to preserve existing levels of safety, as a steadily increasing segment of the driving population experiences diminished capability in terms of a number of relevant aspects of response effectiveness. In addition, conspicuous and comprehensible warning devices should be especially beneficial to elderly drivers in sight-restricted situations. Accordingly, a preservation of highway design adequacy and an improvement in motorist information are the goals of the recommendations in this section.

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D. Design Element: Passing Zone Length, Passing Sight Distance, and Passing/Overtaking Lanes on Two-Lane Highways

Table 30. Cross-references of related entries for passing zone length,
passing sight distance, and passing/overtaking lanes on two-lane highways.

The safety and effectiveness of passing zones depend upon the specific geometric characteristics of the highway section, as well as on how drivers receive and process information provided by signs and pavement markings, integrate speed and distance information for opposing vehicles, and control their vehicles (brake and accelerate) during passing maneuvers. As the number of older drivers in the population increases dramatically over the years 1995­2025, many situations are expected to arise where not only the slower-moving vehicle, but also the passing vehicle, is driven by an older person.

The capabilities and behavior of older drivers, in fact, vary with respect to younger drivers in several ways crucial to this discussion. Studies have shown that while driving speed decreases with driver age, the sizes of acceptable headways and gaps tend to increase with age. While motivational factors (e.g., sensation seeking, risk taking) have been shown to play a major role in influencing the higher speeds and shorter headways accepted by young drivers, they seem to play a less important role in older driver behavior. Instead, the relatively slower speeds and longer headways and gaps accepted by older drivers have been attributed to their compensating for decrements in cognitive and sensory abilities (Case, Hulbert, and Beers, 1970; Planek and Overend, 1973).

The ability to judge gaps when passing in an oncoming lane is especially important. For some older drivers, the ability to judge gaps in relation to vehicle speed and distance is diminished (McKnight and Stewart, 1990). Depth perception—i.e., the ability to judge the distance, and changes in distance, of an object—decreases with age (Bell, Wolfe, and Bernholtz, 1972; Henderson and Burg, 1973, 1974; Shinar and Eberhard, 1976). A recent study indicated that the angle of stereopsis (seconds of visual 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 a group of drivers ages 18­55 to achieve the same level of performance (Staplin, Lococo, and Sim, 1993). McKnight and Stewart (1990) reported that the inability to judge gaps is not necessarily associated with a high accident rate, to the extent that drivers can compensate for their deficiencies by accepting only inordinately large gaps. This tactic has a negative impact on operations as traffic volumes increase, however, and may not always be a feasible approach.

Judging in-depth motion is made difficult by the fact that when no lateral displacement occurs, the primary depth cue is the expansion or contraction of the image size of the oncoming vehicles (Hills, 1980). Studies of crossing-path crashes, where gap judgments of oncoming vehicle speed and distance are critical as in passing situations, indicate an age-related difficulty in the ability to detect angular movement. In laboratory studies, older persons required significantly longer to perceive that a vehicle was moving closer (Hills, 1975). Staplin and Lyles (1991) reported research showing that, relative to younger drivers, older ones underestimate the speed of approaching vehicles. Similarly, 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. Older persons also apparently accept a gap to cross in front of an oncoming vehicle that is a more-or-less constant distance, regardless of the vehicle's speed. Analyses of judgments of the "last possible safe moment" to cross in front of an oncoming vehicle showed that older men accepted a gap to cross at an average constant distance, whereas younger men allowed a constant time gap and thus increased distance at higher speeds (Hills and Johnson, 1980). A controlled field study showed that older drivers waiting (stationary) to turn left at an intersection accepted the same size gap regardless of the speed of the oncoming vehicle (48 km/h and 96.5 km/h [30 mi/h and 60 mi/h]), while younger drivers accepted a gap that was 25 percent larger for a vehicle traveling at 96.5 km/h (60 mi/h) than their gap for a vehicle traveling at 48 km/h (30 mi/h) (Staplin et al., 1993).

Consistent with the AASHTO operational model (AASHTO, 1994), passing sight distance is provided only at places where combinations of alignment and profile do not require the use of crest vertical curves. For horizontal curves, the minimum passing sight distance for a two-lane road is about four times as great as the minimum stopping sight distance at the same speed (AASHTO, 1994). By comparison, the MUTCD defines passing sight distance for vertical curves as the distance at which an object 1,070 mm (3.5 ft) above the pavement surface can be seen from a point 1,070 mm (3.5 ft) above the pavement. For horizontal curves, passing sight distance is defined by the MUTCD as the distance measured along the centerline between two points 1,070 mm (3.5 ft) above the pavement on a line tangent to the embankment or other obstruction that cuts off the view of the inside curve (MUTCD, 1988). The length of passing zones or the minimum distance between successive no-passing zones is specified as 122 m (400 ft) in the MUTCD. As Hughes, Joshua, and McGee (1992) pointed out, the MUTCD sight distance requirements were based on a "compromise between a delayed and a flying passing maneuver," traceable back to the AASHTO 1940 policy that reflected a "compromise distance based on a passing maneuver such that the frequency of maneuvers requiring shorter distances was not great enough to seriously impair the usefulness of the highway."

The basis for the minimum length of a passing zone (122 m [400 ft]) is unknown, however, because research has indicated that for design speeds above 48 km/h (30 mi/h) the distance required for one vehicle to pass another is much longer than 122 m (400 ft) (Hughes et al., 1992). Weaver and Glennon (1972) reported that, in limited studies of short passing sections on main rural highways, most drivers do not complete a pass even within a 244-m (800-ft) section; and use of passing zones remains very low when their length is shorter than 274.3 m (900 ft). Not surprisingly, it has been mentioned in the literature (Hughes et al., 1992) that the current AASHTO and MUTCD passing sight distance values are probably too low. Several studies have indicated that both the MUTCD and AASHTO passing sight distances are too short to allow passenger cars to pass trucks and for trucks to pass trucks (Donaldson, 1986; Fancher, 1986; Khasnabis, 1986).

Several research studies have been performed that have established and evaluated passing sight distance values for tangent sections of highways. As early as 1934, the National Bureau of Standards measured the time required for passing on level highways during light traffic and found that the time to complete the maneuver always ranged between 5 and 7 s regardless of speed. Passing maneuvers were observed at speeds ranging from 16 to 80 km/h (10 to 50 mi/h). They concluded that 274.3 m (900 ft) of sight distance was required for passing at 64 km/h (40 mi/h). Harwood and Glennon (1976) reported that drivers are reluctant to use passing zones under 268 m (880 ft). They recommended that design and marking standards should be identical and include both minimum passing sight distances and minimum length of passing zones, with minimum passing sight distance values falling between the AASHTO and MUTCD values. Kaub (1990) presented a substantial amount of data on passing maneuvers on a recreational two-lane, two-way highway in northern Wisconsin. Under low and high traffic volumes, he found that 24­35 percent and 24­50 percent, respectively, of all passes were attempted in the presence of an opposing vehicle; the average time in the opposing lane (96 km/h [60 mi/h]) was 12.2 s under low-traffic conditions and 11.3 s with high-traffic volumes.

Passing lanes, also referred to as overtaking lanes, are auxiliary lanes provided on two lane highways to enhance overtaking opportunities. Harwood, Hoban, and Warren (1988) reported that passing lanes provide an effective method for improving traffic operations problems resulting from the lack of passing opportunities due to limited sight distance and heavy oncoming traffic volumes. In addition, passing lanes can be provided at a lower cost than that required for constructing a four-lane highway. Based on Morrall and Hoban (1985), the design of overtaking lanes should include advance notification of the overtaking lane; a KEEP RIGHT UNLESS OVERTAKING sign at the diverge point; advance notification of the merge and signs at the merge; and some identification for traffic in the opposing lane that they are facing an overtaking lane. They reported that there is general agreement that providing short overtaking lanes at regular spacing is more cost-effective than providing a few long passing lanes. This feature becomes increasingly attractive as the diversity of driving styles and driver capability levels grows, with faster motorists taking unnecessary chances to overtake slower-moving vehicles.

Finally, although the minimum passing sight distances specified by AASHTO are more than double that specified by the MUTCD, and are based on observations of successful car passing-car observations, Hughes et al. (1992) commented that the model does not take into account the abortive passing maneuver, nor does it consider the length of the impeding vehicle. Saito (1984) determined that the values specified by the MUTCD for minimum passing distance are inadequate for the abortive maneuver, while Ohene and Ardekani (1988) asserted that the MUTCD sight distance requirements are adequate for the driver to abort if the driver decelerates at a rate of 3.2 m/s² for a 64-km/h passing speed (10.5 ft/s² for a 40-mi/h passing speed) and at a rate of 3.9 m/s² for a passing speed of 80 km/h (12.8 ft/s² for a 50-mi/h passing speed). Worth noting is work by Lyles (1981) on passing zone traffic control devices showing that aborted passes could be reduced by more judicious use of passing zone signs. In any event, it cannot be assumed that drivers will always use the maximum acceleration and deceleration capabilities of their vehicles, particularly older drivers.

The age differences in driver capability and behavior noted earlier—i.e., age-related difficulties in judging gaps and in increased perception-reaction time, coupled with slower driving speeds—support a recommendation for passing zone length that is consistent with the upper range of the time and distance values for passing maneuvers reported in this discussion. A recommendation for minimum passing sight distance (MUTCD definition), by comparison, may be keyed to the time required to perceive the need and execute appropriate vehicle control movements to abort a passing maneuver and return to one's own lane. This distance may therefore be smaller than the minimum passing zone length, but should allow an exaggerated perception-reaction time (5 s) to accommodate age-related declines in depth perception and a sufficient interval (3 s) for a smooth lane-change maneuver at passing speeds up to 96 km/h (60 mi/h). In addition, it appears reasonable to recommend a treatment to improve drivers' preview of the end of a passing zone, to facilitate older drivers' decisions and responses in situations where safe operations dictate that they should abort a passing maneuver. Finally, a recommendation to implement passing/overtaking lanes may be justified in terms of overall system safety and efficiency.

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