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Publication Number: FHWA-RD-97-135
Date: January 1998
Older Driver Highway Design Handbook
A. Advance Signing for Lane Closure(s)
A. Design Element: Advance Signing for Lane Closure(s)
Table 31. Cross-references of related entries for advance signing for lane closure(s).
The requirements for safely negotiating a lane closure are an awareness of a decrease in pavement width ahead, and of the direction of the lateral shift in the travel path; a detection of traffic control devices marking the location of the lane drop (beginning of taper); a timely decision about the most appropriate maneuver, taking other nearby traffic into account; and smooth vehicle control through maneuver execution. In the vicinity of a lane closure, the longer the information needs supporting these requirements remain unmet for the least capable drivers within the traffic stream, the less likely is a smooth transition through the work area for all drivers (Goodwin, 1975). The more time that is required for older drivers to prepare and initiate a merging maneuver, the more dense following traffic (including the adjacent lane) is likely to become; this, in turn, will make gap judgments and maneuver decisions at the point of a lane closure more difficult, and will increase the likelihood of erratic vehicle movements resulting in conflicts between motorists.
Relevant alterations in older adults' cognitive-motor processes include: (1) failure to use advance preparatory information (Botwinick, 1965); (2) difficulty in processing stimuli that are spatially incompatible (Rabbitt, 1968); (3) initiation deficit in dealing with increased task complexity (Jordan and Rabbitt, 1977); and (4) inability to regulate performance speed (Rabbitt, 1979; Salthouse, 1979; Salthouse and Somberg, 1982). Stelmach, Goggin, and Garcia-Colera (1987) found that older adults showed disproportionate response slowing when compared with younger subjects, when there was low expectancy for a required movement. 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 such 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 which action to take. Increased viewing time will reduce response uncertainty and decrease older drivers' RT.
In focus group discussions consisting of 81 drivers ages 6586, pavement width transitions were identified as sources of difficulty by 50 percent of the participants (Staplin, Harkey, Lococo, and Tarawneh, 1997). The drivers participating in these discussions suggested longer merging areas to give them more opportunity to find a safe gap and the use of multiple warning signs to allow them to plan their maneuver at an earlier point upstream. Use of multiple signs to give advance notice of downstream work zones and of required maneuvers was also offered as a desired change by older drivers participating in an earlier focus group (Staplin, Lococo, and Sim, 1990).
Lyles (1981) conducted studies on two-lane rural roads to evaluate the effectiveness of alternate signing sequences for providing warning to motorists of construction and maintenance activities that required a lane closure. The signs tested included a standard MUTCD warning sequence, the same sequence augmented with continuously flashing warning lights on the signs, and a sequence of symbol signs (WORKER and RIGHT LANE CLOSED). The most effective sign sequence was one that was flasher augmented; this treatment was twice as effective as similar signs with no warning lights in slowing vehicles in the vicinity of the lane closure.
The use of verbal (text) signing in highway work areas raises sign legibility issues for older drivers. In research conducted to improve the legibility of the RIGHT/LEFT LANE CLOSED and ROAD CONSTRUCTION series signs using test subjects in three age groups (1844, 4564, and 65 and older), Kuemmel (1992) concluded the following: (1) signs that increased both letter size and stroke width (SW) while maintaining or increasing the standard alphabet letter series resulted in the best improvement; (2) increasing letter size while decreasing the alphabet series (e.g., from C to B) reduces sign legibility, particularly at night; (3) the use of letter series E, with its 21-percent increase in SW-to-letter height over 200-mm (8-in) series C letters, appears to overcome the problems of irradiation (or overglow phenomenon) with high-intensity retroreflective materials, thus increasing night legibility; (4) the legibility distance of the ROAD CONSTRUCTION signs can be increased by changing the word "construction" to "work," and increasing the letter size from 175-mm series C to 200-mm series C (7-in C to 8-in C); and (5) for the RIGHT LANE CLOSED series, use of symbol signs will have to supplement word legend signs, and for the CENTER LANE CLOSED series, redundancy of sign placement will have to be employed if a 1,200-mm (48-in) maximum sign size is to be maintained. The author pointed out that the minimum required visibility distance (MRVD) for both signs is 101 m at 88 km/h, and 112 m at 104 km/h (331 ft at 55 mi/h and 369 ft at 65 mi/h). The legibility distances obtained in this study for the current standard construction work zone signs ranged from 198 m (650 ft) for the best observers to 43 m (140 ft) for the worst observers. In addition, 85th percentile values were closer to the minimum legibility distances than they were to the maximum legibility distances. This finding reinforces the need for redundant signing during the approach to a work zone.
Finally, a number of studies performed to determine the effectiveness and motorist comprehension of static signs and variable message signs (VMS's)—also referred to as changeable message signs (CMS's)—for lane closures have been reported. A general indication of the importance of VMSs to accomplish lane control in advance of work zones is provided by a field study on a four-lane section of I-35 in San Antonio conducted by Dudek, Richards, and Faulkner (1981) to evaluate the effects of VMS messages on lane changes at a work-zone lane closure. The measure of effectiveness used to evaluate the VMS was the percentage of vehicles that remained in the closed (median) lane as traffic progressed toward the cone taper. The results indicated that the VMS did encourage drivers to vacate or avoid the closed lane, compared with driver responses at the same site without use of the VMS. The percent volumes in the closed lane were significantly lower when a lane-closure message was displayed than during periods when the sign was blank. Specifically, there was a 46 percent greater reduction in the lane volume attributable to the VMS.
During the conduct of field studies for NCHRP project 3-21(2), the relative proportions of traffic in the through and closed lanes approaching construction lane closures were observed for a sample of more than 196,500 vehicles (Transportation Research Board, 1981). Data gathered in Georgia, Colorado, and California were used to compare these lane distributions between baseline (no VMS) conditions and various VMS applications. A fourth data set, gathered in South Carolina, was used to determine relative effects between certain VMS message alternatives (i.e., speed and closure, speed and merge, closure and merge advisories), and various placement configurations (i.e., one VMS at 610 m [2,000 ft] in advance; or one VMS at 1,207-m [3,960-ft] advance placement; or two VMS devices, one at each advance location; or one VMS placed at 1,207 m [3,960 ft] in advance of the taper and an additional arrow panel at the 610-m [2,000-ft] location). Findings indicated increased preparatory lane change activity, smoother lane-change profiles, and significantly fewer "late exits" (exit from a closed lane within 30.5 m [100 ft] of closure) in locations where a VMS was applied at the 1,207-m (3,960-ft) advance location and an arrow panel at the 610-m (2,000-ft) location.
A recent human factors laboratory study was conducted to determine which VMS message alternatives would be most likely to enhance motorists' compliance with lane control messages in work zones (Gish, 1995). The subjects were divided into two age groups consisting of 24 subjects each: the youngest drivers had a mean age of 23.1 years (range = ages 1633), and the oldest drivers had a mean age of 70.2 years (range = ages 6584). The results of this study indicated that older drivers were more likely to reduce their speed and change lanes than the younger drivers, and that both older and younger drivers' compliance with lane-change messages was strongly influenced by surrounding vehicles and by the visibility of the lane closures themselves, which exerts a strong influence on message credibility. Other factors, such as traffic density, static displays, and merge arrows (arrow panels), influence driver compliance with VMS messages. To optimize lane-change compliance, Gish (1995) recommended that static displays, merge arrows, and other devices be used in addition to VMS messages. A need to study the long-term effectiveness on nonstandard messages was also indicated, and potential improvements in work-zone safety and operations through the use of condition-responsive (real-time) traffic control systems that provide continuously updated information to motorists (for enhanced credibility) were identified.
Table 32. Cross-references of related entries for variable (changeable) message signing practices.
The effectiveness of variable message signs (VMS's), gauged in terms of observable driver behaviors that traffic management procedures are designed to elicit, rests upon a set of reasonably well-understood human factors. A motorist information system must be rational, relevant, and reliable. Driver sensory/perceptual and cognitive capabilities must be thoughtfully considered to ensure that a message will be acquired and then understood, recalled, and applied by the driver within a desired timeframe; the message must seem to clearly apply to the driver and to reflect current conditions to be credible; and it must be accurate in describing what the driver experiences downstream. The credibility of a highway advisory message certainly depends in part upon a presentation strategy that is "rational," but it also must be perceived to be relevant to the individual motorist, and reliable to the point of being virtually error-free. Reliability requirements—being dependent on real-time data on operations as input to the traffic control system—are most difficult to meet, but probably the most important if high rates of compliance in drivers' vehicle control decisions are ever to be realized.
A motorist's ability to use highway information is governed by: (1) information acquisition, or how well the source can be seen or heard and (2) information processing, or the speed and accuracy with which the message content can be understood, and its ease of recall by the motorist after message presentation is completed.
In the acquisition of VMS information, a visual task, the key factors are: (1) its conspicuity, or "attention-getting value" to the motorist; (2) the size, brightness (contrast), stroke width-to-height ratio, and spacing of individual characters of text, which together determine the legibility of the message; (3) the placement of the VMS device—overhead versus one side versus both sides of the highway—which affects its likelihood of being blocked from a motorist's view by other vehicles, as well as the "eyes away from the road" time required to fixate upon the message; and (4) the exposure time, or available viewing time, of each message phase presented on a VMS.
Conspicuity is generally not a problem for any type of VMS under low traffic volumes, although under high volumes with a significant mix of heavy vehicles, a motorist may fail to notice a roadside device because of obscuration. Good conspicuity is achieved by overhead devices under all conditions. The attention value of a VMS display can be maximized by flashing operations, but this also works against information acquisition by reducing exposure time and legibility; this strategy is thus uniformly discouraged for an entire message. In rare circumstances, for a unit of information deemed particularly critical by the highway authority, the flashing of a single text element within a message at a slow rate may be justified. The use of flashing text may help bring the sign to the attention of an older driver who has a reduction in his/her useful field of view and may otherwise fail to notice the sign. If it is standard policy to leave the signs blank, then the mere display of a message will capture the driver's attention. However, if the VMS in question always has some type of message displayed, then slowly flashing (e.g., two cycles per phase) the problem statement line only may be warranted to attract attention. A preferred strategy under such circumstances would be to activate a flashing warning light separate from, though clearly attached to, the VMS.
The legibility of a VMS is influenced by the same factors influencing character and message legibility of static signs, including the key factor of driver visual performance capability. Letter acuity declines during adulthood (Pitts, 1982) and older adults' loss in acuity is accentuated under conditions of low contrast, low luminance, and where there is crowding of visual contours (Sloane, Owsley, Nash, and Helms, 1987; Adams, Wong, Wong, and Gould, 1988). In any event, the legibility for current VMS's is determined primarily by the technology and the device configuration (numbers of rows, characters per row, and number, size, and spacing of pixels per character) as fabricated by a given manufacturer, and for all practical purposes can be treated as a fixed factor—modified by environmental considerations—in considering whether a particular system as implemented in the field will meet motorists' needs.
For any given speed, older drivers' needs dictate a legibility distance that permits the entire VMS message to be read twice in its entirety. As a general rule, at least 305 m (1,000 ft) of legibility distance for a motorist with 20/40 visual acuity should be provided on a 88-km/h (55 mi/h) facility. Of the studies that assessed various character matrix forms (number of elements per character cell), most found a 7 x 9 element matrix to be necessary when using lowercase letters, because of the descenders and ascenders, but a 5 x 7 font was generally deemed acceptable with uppercase-only lettering. The MUTCD specifies a minimum legibility requirement of 198 m (650 ft) under both day and night conditions for Portable Variable Message Signs. Given that the most common format for a portable sign is 450-mm (18-in) tall characters arranged in three lines of eight characters, this provides for a legibility distance of 0.44 m/mm (36 ft/in) of letter height. Thus, letter sizes of at least 450 mm (18 in) should be used to accommodate older drivers' diminished visual acuity. Other variables found to significantly effect VMS legibility for older observers are font, letter width-to-height ratio, contrast orientation, letter height, case, and stroke width (Jenkins, 1991; Mace, Garvey, and Heckard, 1994). The most consistent finding across studies evaluating VMS design elements was that the results found for older drivers were quantitatively but not qualitatively different from those of their younger counterparts. That is, if a manipulation of a variable resulted in an improved score for younger observers, it almost invariably improved older observer performance.
The "target value," legibility, and viewing comfort of light-emitting diodes (LED's) and fiber-optic VMS technologies were compared with flip-disk and conventional overhead guide signs in a field study conducted by Upchurch, Baaj, Armstrong, and Thomas (1991). Younger (ages 1831) and older (ages 6079) subjects in this study demonstrated mean daytime target values for fiber-optic, LED, and flip-disk technologies that all were significantly better (longer) than the values for conventional overhead signs. Under nighttime conditions, however, the poorest performance (shortest distances) were demonstrated by both age groups for the flip-disk technology, falling below the conventional sign values as well. The fiber-optic and LED signs again exceeded the conventional signs, based on nighttime mean target value, with the fiber-optic technology showing a slight superiority for older drivers. Under backlight (sun behind sign) and washout (sun behind driver) conditions, target values for all sign types decreased substantially and the differences among sign types diminished, but the fiber-optic technology still resulted in the best overall performance, across age groups.
Legibility distance results tended to favor the conventional signs, followed by the fiber-optic signs, LED signs, and flip-disk technology. Mean daytime legibility distances for each sign type in this study were as follows: fiber-optic—0.74 m/mm (61 ft/in); LED—0.51 m/mm (42 ft/in); flip-disk—0.47 m/mm (39 ft/in); and conventional—1.07 m/mm (88 ft/in). Under nighttime conditions, the conventional signs again could be read at the longest mean distances, followed closely by the fiber-optic and LED signs, with the flip-disk technology showing the poorest performance. Backlight conditions favored the fiber-optic technology, and washout conditions favored the conventional signs; in both cases, however, the flip-disk technology resulted in the shortest legibility distances. Using a threshold for minimal acceptable legibility distance of 191 m (628 ft), the study concluded that flip-disk signs are deficient under all conditions except midday daytime viewing, LED signs are deficient under both backlight and washout sun conditions, and fiber-optic signs are deficient only with the sun glare present under backlight conditions.
Mean discomfort ratings were consistent with these patterns of results. Fiber-optic and conventional signs were assigned the best (lowest discomfort) ratings under daytime conditions, by younger and older drivers alike. LED signs caused slightly more discomfort for older subjects, and flip-disk signs resulted in the highest discomfort ratings, especially for older drivers. Under nighttime conditions, only the flip-disk technology resulted in high discomfort ratings. Discomfort ratings were more even, and much higher, across sign types under backlight conditions where the sun was behind the sign, though flip-disk signs still were rated the worst by both age groups. Under washout conditions, subjects reported little discomfort for either the fiber-optic or conventional signs, but much greater and roughly equivalent levels of discomfort with the LED and flip-disk technologies.
Table 33 contains legibility distances from the Upchurch et al. (1991) study. For older drivers, the legibility distances are lower due to the well-documented degradation of visual performance with age. Unfortunately, this is the only study that has assessed legibility distances for older observers. The legibility distances for conventional bulb matrix and LED/flip-disk hybrid VMS's were estimated from the results of the Upchurch data and data cited in Dudek (1991).
Table 33. Day and night predicted legibility distances (ft) for various sign technologies.
* Legibility distance of this technology decreases over time, because as LED's age, they become less bright.
1 ft = 0.305 m
Although the bulb matrix VMS was assessed by Upchurch et al. (1991), no legibility distances for that sign were reported. Legibility distances for this type of VMS have been obtained; however, it is unknown whether any older observers have been used in assessing legibility distances. Dudek (1991) cited a study in which bulb matrix VMS's provided legibility distances of 244 m (800 ft) during the day and 229 m (750 ft) at night. These distances are similar to the legibility distances obtained by Upchurch et al. (1991) for LED-type VMS's using younger observers. Until psychophysical data can be obtained for older observers viewing bulb matrix signs, the legibility distances for older observers are assumed to be roughly 204 m (671 ft) during the day and 173 m (569 ft) at night. These estimates are based on applying the ratio of older-to-younger legibility distances for the LED-type display.
VMS placement affects information acquisition under heavy traffic conditions where a center lane driver's view of a roadside device may be obscured for lengthy intervals. If a facility has more than two lanes, a consideration may be given to placement of a portable VMS in the median—space permitting and where glare from opposing vehicles is absent or minimal due to a large glare angle—rather than on the right shoulder, since lane control practices for heavy trucks are common throughout many corridors.
A motorist's reading time for a VMS message dictates the required exposure time at a given speed. Exposure time is the length of time a driver is within the legibility distance of the message. The minimum recommended exposure time per page (phase) for a three-line VMS is 3 s, aside from a consideration of any particular set of driver characteristics. However, while some jurisdictions have selected briefer exposure times, the increasing numbers of older drivers on limited-access highways makes an even stronger case for the 3-s minimum per page. Reading time is the time it actually takes a driver to read a sign message. In instrumented vehicle studies conducted in light traffic with familiar drivers on a rural freeway, reading times averaged 1 to 1.5 s per unit of information (Mast and Ballas, 1976). Reading times under "loaded" driving conditions would be higher, such as under extreme geometry, heavy traffic volumes, large volume of truck traffic, traffic conflicts, or poor climatological conditions. More recent field research using unfamiliar drivers has indicated that a minimum exposure time of 1 s per short word (four to eight characters) or 2 s per unit of information, whichever is larger, should be used (Carvell, Turner, and Dudek, 1978; Messer, Stockton, and Mounce, 1978; Weaver, Richards, Hatcher, and Dudek, 1978; Dudek, Huchingson, Williams, and Koppa, 1981). A unit of information is a data item given in a message, that can answer one of the following questions: (1) what happened? (2) where? (3) what is the effect on traffic? (4) for whom is the advisory intended? and (5) what driver action is advised? Thus, the exposure time for a three-line message could vary from 3 s to as much as 6 s, with each phase of a portable VMS at the lower end of this range and with each permanent VMS phase (page) at the upper end, due to differences in the number of characters per line. Reducing the exposure time per phase is warranted only when information is being repeated. For example, a three-line message may be displayed for only 2.5 s if it is a second phase of a two-phase message which repeats one or two lines from the first phase. If the second phase presents new information, the recommended minimum exposure time for both phases remains 3 s.
For a given operating speed, exposure will increase with increasing legibility distance. For example, an overhead sign message legible at 198 m (650 ft) will be exposed to drivers traveling at 88 km/h (55 mi/h) for approximately 8 s. With a legibility distance of 305 m (1,000 ft), the message will be exposed for about 12 s. Legibility distances for portable VMS's vary from the minimum of 198 m (650 ft) specified by the MUTCD Part VI and American Traffic Safety Services Association (ATSSA) to over 305 m (1,000 ft), depending on the technology. Permanent VMS's generally have legibility distances in the higher range of 274366 m (9001,200 ft). However, there is a point at which a sign becomes unreadable during a driver's approach to a VMS, which reduces the legibility distance, particularly for side-mounted VMS's. This unreadable distance, which is dependent on the number of lanes and the sign technology, as well as how far the sign is placed from the roadway edge or how high above the roadway it is mounted, ranges from 85 m to 128 m (280 ft to 420 ft). In an existing system, therefore, required exposure times dictate the maximum length of message that can be displayed, and in all cases, it is desirable that motorists be able to read the entire message on an (unobstructed) VMS twice.
For these reasons, the maximum number of phases used to display a message on a permanent VMS should be two. The most effective format for VMS message presentation is a single phase which consists of a maximum of three units of information, but if two are required, each should be worded so that it can stand alone and still be understood. Portable VMS devices, though limited to fewer characters per line, should also be restricted to two-phases. At high speeds (88 km/h [55 mi/h]), a driver may only have 2.8 to 4.6 s to read a message on a side-mounted VMS, depending on the available legibility distance. For this reason, messages should be restricted to one phase at high speeds.
The motorist's need for rapid understanding and integration of message components also focuses attention on the formatting of multiword text displays. The main concern is with "units of information"—i.e., where and how to divide phrases—and with the use of abbreviations and contractions in VMS messages.
Work zones constitute driving situations that require a high amount of controlled processing, and data show that cognitive ability scores that measure processing efficiency decline with age (Ackerman, 1987). In fact, sensory memory, working memory, and divided attention all show a decline with aging and must be considered in the display of messages on VMS's. Sensory memory is a high-capacity, briefly accessible register from which information is lost through decay or interference. While there is evidence that older adults require slightly longer to establish a legible "icon" in sensory memory, another set of findings suggests that with advancing age, images are instead more susceptible to masking by other (successive) stimuli (Walsh, Till, and Williams, 1978; Cerella, Poon, and Fozard, 1982). This suggests that a message should be limited to a single phase, or certainty no more than two, because multiple phases will interfere with message comprehension. There is also considerable evidence that older adults have poorer working memory function than younger adults (Salthouse, 1991; Salthouse and Babcock, 1991). This suggests that message length be limited to the fewest, most relevant units possible. Finally, older adults are particularly disadvantaged when they are required to use working memory to manage multiple tasks (Ponds, Brouwer, and van Wolffelaar, 1988). Van Wolffelaar, Brouwer, and Rothengatter (1990) found that older drivers made more tracking (steering) errors when required to attend and respond to a dot-counting task and a task that required them to monitor peripheral events. In this study, older adults also showed a dramatic increase in the rate of nonresponding on the dot-counting task under multiple task conditions, compared to younger subjects. Van Wolffelaar et al. (1990) concluded that there is a disproportionately greater problem for older adults in divided attention situations and directly linked this to a higher accident rate for older adults in time-pressured, complex traffic situations.
The minimum required information for traffic management includes: (1) a statement of the problem and (2) the action statement(s)—i.e., a driver needs to know what to do and one good reason for doing it. Additional elements are included as needed for a specific situation. The key here is not to burden the driver with unnecessary information. Only about two-thirds of drivers are able to recall completely four pieces of information (problem, effect, attention, and action); however, 8090 percent can recall the action message (Huchingson, Koppa, and Dudek, 1978). Two problems in message presentation must be avoided: (1) providing too much information in too short a time and (2) providing ambiguous information that leaves either the intent of the message or the desired driver response uncertain.
The first problem does not refer solely to reading time difficulties, as discussed above; instead, it refers to the number of ideas, or "information units," contained in a message. Certainly, the number of words displayed on a sign is important, but so is the manner in which words are grouped. Units containing one word (DELAY), two words (DELAY AHEAD), or many words (MAJOR DELAY AT HIGH STREET) are equally difficult to remember when the display is no longer in sight. However, a series of, say, six units of information in a message displayed on a permanent VMS will be easier to remember if presented in two phases of three units each than if all six units are presented on a single phase. Studies have concluded that no more than three units of information should be displayed on one sequence when all three units must be recalled by drivers (Huchingson et al., 1978; Dudek et al., 1981; Gish, 1995).
Gish (1995) conducted a human factors laboratory study addressing the perceived timeliness, accuracy, and credibility of VMS messages using both younger (ages 1633) and older (ages 6584) test subjects. Results showed that correct recall of the first VMS phase (a downstream speed advisory) was nearly 100 percent for both age groups. However, successive phases of information (containing downstream delay and route diversion information) were recalled less accurately. For the delay information (second phase), correct recall for the younger subjects was about 82 percent, versus 60 percent for the older subjects. For route numbers (third phase), correct recall was 55 percent for the younger subjects and 19 percent for older subjects. These results reinforce the earlier recommendation that a maximum of two phases should be used.
When a message must be divided into two phases, it is desirable to repeat key words from the first phase on the second phase, to provide assurance that all drivers see the message at least once. This also allows information rehearsal, as provided by an additional "learning trial," which will facilitate message recall when the device is no longer in sight. A recommended standard practice is therefore to put the problem on line 1, the location on line 2, and alternate either the effect and action or diversion information on line three, repeating lines 1 and 2 on both phases.
The second type of problem can occur when an unfamiliar word or abbreviation is used, when a word is hyphenated or a phrase is divided inappropriately, or when an abbreviation or a word can mean different things in different word pairings or contexts. Ambiguity occurs, for example, when CENTER LANE is used on a freeway with four or more lanes in one direction. Another example is the use of LANE CLOSED versus LANE BLOCKED, to denote a prolonged closure for construction/maintenance versus a temporary blockage due to an accident or stall. To foster the most simple and consistent practice for motorists, LANE CLOSED is recommended under both roadwork and incident conditions, because at the time the message is displayed, the lane is effectively closed. Finally, neither FREEWAY BLOCKED nor FREEWAY CLOSED should ever be used when at least one lane is open to traffic.
Abbreviations also have the potential to be misunderstood by some percentage of drivers, exacerbating message comprehension problems for individuals with (age-related) diminished capabilities. It has been determined that certain abbreviations are understood by at least 85 percent of the driving public independent of the specific context (e.g., BLVD = boulevard). A second category of abbreviations are understood by at least 75 percent of the driving population but only with a prompt word, (e.g., LOC means "local" when shown with "traffic"). Other abbreviations are prone to be frequently confused with another word (e.g., WRNG could mean either "warning" or "wrong") and should be avoided. Following are lists of abbreviations in three categories, extracted from Dudek et al. (1981): (1) those that are acceptable (understood by at least 85 percent of the driving population) when shown alone (table 34); (2) those that are not acceptable and, therefore, should not be used (table 35); and (3) those that require a prompt word (table 36). Table 34 also includes abbreviations taken from the MUTCD, as well as common contractions used in the English language.
Table 34. "Acceptable" abbreviations for frequently used words.
Table 35. Abbreviations that are "not acceptable."
Source: Dudek, Huchingson, Williams, and Koppa (1981).
* Prompt word should precede abbreviation.
C. Design Element: Channelization Practices
Table 37. Cross-references of related entries for channelization practices.
Channelization systems include the use of cones, posts, tubes, barricades, panels, drums, amber-flashing and steady-burn lights, and standard and raised/recessed pavement markings. They are used to direct motorists into the open lanes and to guide them through the work area. They must provide a long detection distance and be highly conspicuous under both day and night conditions. Using data collected by the police, it has been estimated that anywhere from 80 to 86 percent of the accidents in work zones can be attributed to driver error (Nemeth and Migletz, 1978; Hargroves and Martin, 1980). Hargroves and Martin (1980) found that accidents with fixed objects within a work-zone account for a greater percentage than other accident types, such as rear end or sideswipe. Nemeth and Migletz (1978) found that nighttime accidents are concentrated in the taper area. The most significant problems with channelization in work zones have been identified as: (1) failure to use, or hazardous use of, temporary concrete barriers; and (2) inadequate or inconsistent use of devices and methods in closing roadways and establishing lane-closure tapers (Humphreys, Maulden, and Sullivan, 1979).
Older drivers, like alcohol-impaired and fatigued drivers, show reduced sensitivity to contrast. Olson (1988) pointed out that the brightness of a traffic control device is the main factor in its attention-getting capability: in a visually complex environment, the brightness must be increased by a factor of 10 to achieve conspicuity equivalent to that found in a low-complexity environment. A major problem at night is reduction in contrast sensitivity, which makes it difficult to see even large objects when they cannot be distinguished from their background. Older drivers also have difficulty processing information due to less effective scanning behavior and eye movements, diminished visual field size, difficulty in selective attention, and slower decisionmaking. Inconsistent use of barrels and traffic cones to delineate the travel path may be a particular problem for older drivers, especially when applied in the presence of remnants of old lane markings, because such inconsistency is confusing and older drivers (and inattentive drivers) are not able to react as quickly to conflicting traffic cues (National Transportation Safety Board, 1992). To compensate for their slower information-processing capabilities, their reduced visual capabilities, and their slower reaction time, older drivers often drive more slowly. Although driver age was not studied, Hargroves and Martin (1980) found that slow-moving vehicles were overrepresented in work-zone accidents. Older drivers also show reductions in lane-keeping ability, which is further compromised when they are required to attend to other tasks, in unfamiliar surroundings. Finally, steering abilities may be adversely affected by physical problems such as arthritis.
McGee and Knapp (1979) performed an analytic study to develop a performance requirement/standard for the detection and recognition of retroreflective devices (cones, drums, panels, and barricades) used in work zones. The performance standard developed in this study, presented in terms of visibility requirements (i.e., the distance at which motorists should be able to detect and recognize the devices at night) and established using the principles of driver information needs and the requirement for decision sight distance, calls for a minimum visibility distance of 275 m (900 ft) when illuminated by the low beams of standard automobile headlights at night under normal atmospheric conditions.
Pain, McGee, and Knapp (1981) evaluated the effectiveness of traffic cones and tubes, vertical panels, drums, barricades, and steady-burn lights in laboratory studies, in controlled field studies, and at actual construction sites. Overall, there were no major differences between the device categories in the daytime. At night, barricades, panels, drums, cones, and tubes were also equivalent when the optimized cone and tube reflectorization was used. Posts and cones with 150 mm (6 in) of collar did not elicit an equivalent level of driver behavior, especially at night. Interestingly, in comparing the meaning of chevrons versus stripes, it was found that diagonal, horizontal, and vertical stripes conveyed no consistent directional information; chevrons, though less easily detected than the stripe patterns, effectively and unambiguously indicated that a movement to the left or right was required. Since diagonal, horizontal, and vertical stripes conveyed no consistent direction information, Pain et al. (1981) concluded that there was no reason to have a diagonal stripe pattern for left and right "sidedness." They pointed out, however, that only one direction of diagonal should be allowed in an array so there is always a consistent pattern or image on devices.
In terms of device spacing, comparisons of regular speed-limit spacing (16.8 m [55 ft] in the test), half spacing (8.4 m [27.5 ft]), and double spacing (33.5 m [110 ft]) of Type I barricades and 200-mm x 600-mm (8-in x 24-in) panels showed that changes in spacing produced little impact on driver behavior. There were no significant speed or lateral placement differences between half, regular, and double speed-limit spacing during the day. At night, however, when devices were placed at half spacing, they produced a speed reduction, apparently from the illusion that the motorist was going faster than he or she actually was. Devices placed at double spacing tended not to perform as well as when they were placed at regular speed-limit spacing, as drivers made lane changes and detected arrays of traffic control devices sooner with shorter spacings. From these findings, Pain et al. (1981) recommended that: (1) all devices be placed at speed limit spacing for most conditions and, in all cases, along the taper or transition section; (2) if there is no construction work or hazard in the closed lane for a substantial length, or traffic delays, the spacing can be increased to no more than twice the speed limit; and (3) shorter spacings may prove to be useful where speed reduction is desired.
Device-specific findings by Pain et al. (1981) are as follows:
Other findings were reported for comparisons of steady-burn lights and Type II and Type III sheeting. The steady-burn lights provided the longest detection distances at night compared with all other materials, and they more than tripled the distance (or zone) in which lane changing occurred before the taper. In comparisons of Type II sheeting and Type III sheeting on cone and tube optimization tests, Type III was significantly better at night on a flat road. Narrow-angle sheeting, even though offering high brightness, was not effective under certain sight geometry characteristics, such as hills and curves. Type III sheeting and steady-burn lights were comparable in terms of point-of-lane-change and array detection distance; however, the authors noted that the effect of vertical or horizontal curvature must be considered.
There have been mixed results regarding the effectiveness of steady-burn lights in highway work zones. The use of steady-burn lights mounted on channelizing devices has been shown to significantly influence driver behavior in some work-zone configurations, and they are particularly effective in left-lane closures (KLD Associates, 1992). Although older drivers (age 55 and older) consistently showed poorer performance than younger drivers in all study conditions, evidence was found that the use of lights improved the performance of older test subjects. The variables manipulated in this study included work-zone configuration (left-lane, right-lane, and shoulder closures), device type (panels versus drums), and light placement (every device, alternate devices, no lights). Drivers of all ages were able to identify lane and shoulder closures from greater distances when lights were used on channelization devices, as opposed to when the channelizing devices were used alone. Steady-burn lights produced a higher percentage of correct responses (determining the direction the channelizing devices were leading) for all driver age groups when used in left-lane closures than in right-lane closures. Interestingly, the use of lights on every other drum or vertical panel (placement on alternate devices) generated more correct responses than the use of lights on consecutive devices. More generally, the literature suggests that in environments characterized by high-speed operations, compromised visibility due to inclement weather, and/or complex maneuvers required as a result of work-zone configuration, the deployment of steady-burn lights should be considered on all channelizing devices used for right-lane closures.
However, Pant, Huang, and Krishnamurthy (1992) obtained a different result when they examined the lane-changing behavior of motorists in advance of tapered sections as they drove an instrumented vehicle through work zones during the day, at night when steady-burn lights were placed on drums, and at night when the steady-burn lights were removed. They measured the traffic volume at several locations in each lane in advance of the taper. Results showed that the steady-burn lights had little effect on driver behavior in the work zones studied. It was concluded by Pant et al. that steady-burn lights have little value in work zones that employ drums with high-intensity sheeting and a flashing arrow panel as channelizing devices.
Opiela and Knoblauch (1990) conducted laboratory and field studies to determine the optimal spacing and use of devices for channelization purposes in the taper or tangent sections of work zones. In the laboratory study, the recognition distances of eight different device types, spaced at the standard distance and at 1.5 and 2.0 times the standard distance, were measured for 240 subjects. Results indicated variability between the performance of most channelizing devices across the spacings tested. Right- and left-lane closures were then used at six actual work zones, to test the various device spacings under both day and night conditions. Field data were collected at four points equally spaced over 610 m (2,000 ft) before the work zone and the activity at the start of the taper for the lane closure, according to the premise that the most effective treatment would minimize the percentage of traffic in the closed lane at the start of the taper. Statistical analysis of 2,100 5-minute observation periods showed that neither type of device (round barrels, oblong barrels, Type II barricades, and cones with reflective collars) nor device spacings (16.8, 24.4, and 33.5 m [55, 80, and 110 ft]) had a significant effect on driver lane-changing behavior.
Cottrell (1981) also found that driver lane-change response was not strongly dependent on the channelizing device employed in a work-zone taper. The objective of this study was to evaluate the effectiveness of alternative orange-and-white chevron patterns on vertical panels and barricades that form an arrow pointing in the direction in which traffic is being diverted, compared with traffic cones, simulated drum vertical panels, and Type II barricades and vertical panels with standard orange-and-white striping patterns. The measure of effectiveness was the position of lane changing relative to the transition taper. Although the subjective evaluation revealed that chevron patterns were preferred over the presently used patterns because of their clear directional message, the positions of lane changing were similar for the stripes and chevrons. With respect to the spacing of devices, it was generally found that lane changes occurred more frequently at greater distances from the taper when the devices were spaced every 12 m (40 ft), as opposed to every 24.4 m (80 ft).
In a supplemental test, the effectiveness of the Jersey concrete barrier was compared with that of the channelizing devices studied (Cottrell, 1981). The barrier was marked with steady-burn warning lights and 150-mm (6-in) reflectors and had a slope of 16:1 for the 58.5-m (192-ft) taper. The Jersey barrier was rated equal to the cone during the daytime and lower than all other devices based on the lane-change positions. It was recommended that a supplemental taper of channelization devices be used with the Jersey barrier. In a study of concrete barrier visibility, Pain et al. (1981) found that reflectors were superior to reflectorized tape. Logically, the most conspicuous types of reflective devices, such as those containing cube-corner lenses, will be most effective in this regard.
Overall, Pain et al. (1981) concluded that most devices show relatively successful detection and path guidance performance. However, a major deterrent to effectiveness is not the device itself; poor positioning, dirt, and overturned devices destroy the visual line or path created by the channelizing devices. Therefore, although use of appropriate devices is important, of equal importance is conscientious set-up and care of the work zone.
In terms of the threat posed to drivers by passenger compartment intrusion or interference with vehicle control, or the threat to workers and other traffic from impact debris, plastic drums, cones, tubes, and vertical panels used as channelizing devices presented no hazards in full-scale vehicle crash tests (Bryden, 1990). However, Types I and II barricades and portable signs and supports formed impact debris, which was often thrown long distances through work zones, posing a threat to workers and other traffic. The American Traffic Safety Services Association (ATSSA) is opposed to the use of metal drums in work zones as channelizing devices, as they pose a hazard to motorists as well as workers in the zone (TranSafety, 1987). They suggest the use of plastic drums, which are safer. Riedel (1986) described studies showing that a substantial number of work-zone accidents occur in the taper and the crossover where channelization devices are located. The frequency of accidents involving drums has led to the use of forgiving devices such as plastic drums, which in tests have been shown to be safer than steel drums. Juergens (1972) noted that because barricades are inherently fixed-object hazards, they should not be used as primary delineation to guide traffic. Further, they should not be used unless the construction hazard the motorist may encounter is greater than the hazard of striking the barricades. A concern with the use of steady-burn lights mounted on channelizing devices was highlighted in full-scale vehicle crash tests evaluating the performance of work-zone traffic control devices, where warning lights attached to these devices were thrown free, posing a potential threat to workers and other traffic (Bryden, 1990).
D. Design Element: Delineation of Crossovers/Alternate Travel Paths
Table 38. Cross-references of related entries for delineation of crossovers/alternative travel paths.
Studies have established that: (1) a substantial proportion of construction work-zone accidents occur in the taper and the crossover, where channelizing devices are usually located; (2) darkness is associated with an increase in the frequency of accidents in these areas; and (3) construction zones are associated with increases in the incidence of fixed-object, rear-end, and head-on accidents (Graham, Paulsen, and Glennon, 1977). Nemeth and Rathi (1983), studying accident types in construction zones on the Ohio Turnpike, found that 52.4 percent of the accidents were with fixed objects, and 68.3 percent of the crossover accidents involved collisions with channelizing devices or other objects. In this study, 69.4 percent of the accidents at the first curve of a crossover occurred at night. Nemeth and Migletz (1978) found that 60.7 percent of single-vehicle fixed-object accidents were collisions with drums and 29.8 percent of all accidents involved collisions with drums. They also found that the proportion of accidents involving construction objects (drums) at night is significantly higher than the proportion of daylight construction object accidents. The results of these studies highlight the need for highly conspicuous and properly installed and maintained channelizing devices.
The relationships between functional capabilities of older drivers and their performance that are likely to be of greatest operational significance as they approach and negotiate a crossover in a work zone can be summarized as follows. Age-related declines in acuity (both static and dynamic) and contrast sensitivity will delay recognition of channelizing devices and pavement markings and will delay comprehension of the information provided by advance warning signs. This information loss in the early stages of the driver's vehicle control task will be compounded by attentional and decisionmaking deficits shown to increase with increasing age, with age differences in performance magnified as serial processing demands for conflict avoidance and compliance with traffic control messages increase during the approach to the work zone. Age-related decrements in the useful field of view, selective attention, and divided attention/attention-switching capabilities will slow the initiation of a driver's response when a lane change is required prior to the transition zone, or maneuvering through channelization across the median. In addition, less efficient working memory processes may translate into riskier operations for older drivers in unfamiliar areas if concurrent search for and recognition of navigational cues is required; such demands disproportionately tax "spare capacity" for lanekeeping and conflict avoidance for older operators. Finally, the execution of vehicle-turning movements becomes more difficult for older drivers as bone and muscle mass decrease, joint flexibility is lost, and range of motion diminishes. Simple reaction time, while not significantly slower for older drivers responding to expected stimuli under nominal operating conditions, suffers operationally significant decrements with each additional response to an unexpected stimulus, e.g., as required in emergency situations. In addition, older drivers' increased sensitivity to glare and reduced dark adaptation ability will compound the difficulties described above while driving at night.
The National Transportation Safety Board (NTSB) has expressed concern about the lack of positive separation of opposing traffic in work zones (NTSB, 1992). The NTSB uses "positive barrier," or "positive separation of traffic," to refer to the use of concrete barriers to separate traffic, notably the Jersey-type barrier. (A number of States distinguish between these terms, using "positive separation" to describe various channelization treatments which do not necessarily involve use of a physical [Jersey] barrier.) The NTSB (1992) asserts that, "Accident rates, particularly fatal accident rates, increase significantly when an interstate highway is switched from a four-lane, divided operation to a two-lane, two-way operation (TLTWO) during construction work." Research bearing on the use of channelization and barrier delineation for TLTWO's is described below.
A crossover requires a change in direction and may require a reduction in speed. This requires adequate advance warning of the lane and speed reduction, conspicuous and unambiguous delineation/channelization in the transition zone, and conspicuous separation of opposing traffic the length of the TLTWO. One survey of drivers in Houston and Dallas (Texas) by Hawkins, Kacir, and Ogden (1992) found that only half of the respondents correctly understood that they should turn before reaching the CROSSOVER sign (D13-1) when this device was shown in a field placement in an arterial work zone. Of course, the D13-1 sign panel is identified in the MUTCD as a device used in permanent installations on divided highways, not as a temporary device for use in construction zones. The poor comprehension of motorists for such an explicit message is alarming, nevertheless, and suggests the need for heightened conspicuity of guidance information in this situation. Hawkins et al. recommended that the spacing of channelizing devices be decreased in the vicinity of a crossover to reduce drivers' confusion.
Next, Pang and Yu (1981) conducted a study to verify whether concrete barriers were justified at transition zones adjacent to TLTWO's on normally divided highways, based on accident experience in several construction zone TLTWO's. They found that 34 of the 44 total accidents that occurred in TLTWO's were within the transition zone. Four head-on accidents occurred on two-way, two-lane segments away from the transitions. The transition zone was defined as the roadway section at which traffic flow was converted from a four- to a two-lane operation and vice versa. The absence of opposing traffic precluded the occurrence of head-on accidents during the study period; however, more than one-half of the accidents (56 percent) had the potential of becoming head-on collisions. The authors concluded that on relatively low-volume highways, delineation devices appear to be adequate at transition zones, assuming they are placed properly. A regression analysis provided by Pang (1979) indicated that as annual average daily traffic increases, the accident rate at transition zones also increases, with a concurrent increase in the head-on accident rate at the transition zone.
Project duration and approach speed are two other variables that appear to affect the head-on accident rate at transitions (Pang and Yu, 1981). Graham (1977) concluded that as project duration increases, the accident rate at the transitions decreases. Expectancy issues were highlighted as a plausible explanation. Pang and Yu (1981) reported that because the accident rate in the transition zone increases with shorter project duration, concrete barriers may be necessary for short-term projects. However, long-term projects are expected to have a greater number of accidents owing to a longer period of exposure. Thus, installation of concrete barriers would be more economically justified for long-term projects than for short-term ones. With regard to approach speed, it can be expected that as speed to the transition increases, the chances of a head-on collision would also increase, due to the tendency of vehicles to stray out of their lanes at curves such as those present in transition zones. Pang and Yu (1981) suggested that concrete barriers appear to be justified at transition zones where approach speeds are high.
The conspicuity of concrete safety shaped barriers (CSSB's) is an important issue. Their composition provides little contrast with the roadway pavement, making them difficult to see at night, particularly in the rain, and under opposing headlight glare conditions. Proper barrier delineation treatments will provide drivers with a defined path during darkness and adverse weather conditions. Standard barrier delineation treatments include Type C steady-burn warning lights on top of the barrier, retroreflective devices on the top or side of the barrier, vertical panels placed on top of the temporary concrete barrier, and reflective pavement markings on the side of the barrier. The results of studies of barrier delineation in work zones have been mixed (Ullman and Dudek, 1988). For instance, Mullowney (1978) suggested that delineation should be mounted on the top of the barrier so it will retain its reflectivity longer and require less maintenance. However, Ogwoaba (1986) recommended side-mounted concrete barrier delineation so that the delineators are not masked by oncoming headlight glare. The size and brightness of delineators is another controversial topic, with some studies suggesting the use of larger but less bright devices (Davis, 1983; Bracket, Stuart, Woods, and Ross, 1984; Kahn, 1985) and others recommending smaller, brighter reflectors (Mullowney, 1978; Ogwoaba, 1986). Kahn (1985) found that the delineation of portable concrete barriers improved considerably through the use of cylindrical reflectors on top and smaller units on the side of the barrier at 7.6-m (25-ft) intervals. Delineator spacings ranging from 7.6 m to 61 m (25 ft to 200 ft) have been recommended by various studies.
Ullman and Dudek (1988) conducted a study of five barrier delineation treatments, using observations of driver performance to determine how different delineator types, spacings, and mounting positions on the barrier affect nighttime traffic operating in the travel lane next to the barrier. An additional objective of the study was to determine how the visibility and brightness of different types of delineators deteriorate over time because of dirt and road film; in a controlled field study, drivers ages 1856 were asked to provide subjective evaluations of delineator brightness. The study was not conducted at a work zone, but was conducted on an illuminated urban freeway with four lanes in each direction. The CSSB was located 0.3 m (1 ft) from the inside travel lane. The five delineation treatments were: (1) top-mounted cube-corner lenses at 61-m (200-ft) spacing; (2) side-mounted cube-corner lenses at 15.2-m (50-ft) spacing; (3) top-mounted reflective brackets at 15.2-m (50-ft) spacing; (4) side-mounted reflective brackets at 61-m (200-ft) spacing; and (5) top-mounted reflective cylinders at 15.2-m (50-ft) spacing. The cube-corner reflector (treatments 1 and 2) had a diameter of 82.5 mm (3.25 in). The brackets (treatments 3 and 4) were 76 mm (3 in) high and 108 mm (4.25 in) wide, and were covered with high-intensity sheeting. The cylindrical tube (treatment 5) had a diameter of 76 mm (3 in) and was 152 mm (6 in) high, and was wrapped with high-intensity sheeting. Before-and-after data were obtained for the following measures of effectiveness: lane distribution, lane straddling, and lateral distance from the left rear tire to the bottom of the CSSB.
Results of the driver performance data collected by Ullman and Dudek (1988) showed that the treatments had very little practical effect on lane distribution. Lane-straddling rates at all of the treatment segments were low during the higher volume nighttime hours; however, a significant increase in lane straddling occurred for treatment 2. The data suggested that the combination of close delineator spacing and the side-mounted position may make some drivers too apprehensive of driving near the barrier. Lateral distance data showed significant differences during the higher volume nighttime hours for treatment 4 and treatment 5. Lateral distance distributions shifted away from the barrier at treatment 4 and closer to the barrier at treatment 5. Subjective evaluations for clean delineators showed brightness rankings to be the same for all treatments. Treatments 14 received adequate ratings from at least 80 percent of the subjects, while treatment 5 was rated adequate by only 50 percent of the subjects. With respect to each treatment's relative effectiveness in helping drivers maintain a safe travel path next to the CSSB, the rankings did not differ significantly; however, treatment 5 again received the worst score. Subjects stated that side-mounted delineators were preferable to top-mounted delineators because side-mounted delineation provided a more direct line of sight, a better indication of the location of the wall, and a more realistic perception of the lane width. For dirt-covered delineators, treatment 2 was rated as brightest and most effective, while treatment 5 was rated as dimmest and least effective. Although further research was deemed necessary due to limitations in the study scope and funding, a recommendation made by the study authors based on the delineators studied was to use cube-corner lenses for delineating CSSB's in narrow freeway median applications, because these delineators do not lose their reflectivity due to dirt and grime as quickly as those covered with high-intensity sheeting. In addition, for situations with limited lateral clearance, as is common with TLTWO, top-mounted delineation is recommended, because side-mounted close delineator spacing results in lane straddling if the barrier is located close to the travel lanes. Although subjects indicated a preference for close spacings, driver performance data did not show any differences between 15.2-m (50-ft) and 61-m (200-ft) spacing. The authors recommended that a 61-m (200-ft) spacing be considered maximum, and that closer spacings may be necessary for CSSB's on sharp curves. The recommendations were also deemed appropriate for CSSB's in work zones.
E. Design Element: Temporary Pavement Markings
Table 39. Cross-references of related entries for temporary pavement markings.
Preconstruction centerlines and edgelines that are not obliterated may confuse drivers about the exact locations of lanes. The National Transportation Safety Board (1992) has reported that although guidelines exist for proper signing and striping in construction areas, the traffic control techniques used in many jurisdictions are not in compliance with the guidelines. Lewis (1985) stated that if drivers are presented with conflicting information (as may be the case in a work zone), they will generally choose to follow the pavement, as the pavement itself is a primary source of information for drivers. This points to a need for unambiguous pavement delineation patterns in work zones, to provide clear guidance—particularly at night and under adverse weather conditions—and to accommodate drivers with visual limitations resulting from age, fatigue, or alcohol consumption.
The research findings that have the greatest bearing on age differences in drivers' ability to acquire and use information provided by roadway delineation are a decline in spatial contrast sensitivity and acuity for older drivers, and a general slowing of response related to the specific deficit in visual search ability to rapidly discriminate more important from less important information in a driving scene.
Discrimination of the boundaries of the traveled way often involves only slight differences in the brightness of the road surface versus the shoulder or surrounding land. The ability to obtain such "edge information" depends upon a driver's sensitivity to contrast. Age differences in contrast sensitivity, beginning at approximately age 40 and becoming progressively more exaggerated with advancing age, demonstrate significant decrements in performance for older subjects (Owsley, Sekuler, and Siemsen, 1983). Under constant viewing conditions, older observers have lower contrast sensitivity especially in situations where there is a reduction in ambient light levels. A 60-year-old driver requires 2.5 times the contrast needed by a 23-year-old driver (Blackwell and Blackwell, 1971).
Age decrements in visual search and scanning capabilities are widely reported in gerontological research. Rackoff and Mourant (1979) measured visual search patterns for 10 young (ages 2129) and 13 older (ages 6070) subjects as they drove on a freeway under day and night conditions in low to moderate traffic. They reported that differences between young and older test subjects' performance were most apparent at night, and that older subjects required more time to acquire the minimum information needed for vehicle control. Thus, older drivers require delineation information that is optimal from the standpoints of both attention conspicuity and search conspicuity downstream, and that provides unambiguous path guidance cues for moment-to- moment steering control. Uncertainty about roadway heading and lane position has been cited by older driver focus group members as reasons for driving slower, for erratic maneuvers caused by last-second steering corrections, and for simply avoiding nighttime operations (Staplin, Lococo, and Sim, 1990). An exaggeration of the difficulties older drivers have in rapidly discerning the correct travel path may be expected in construction zones, where drivers must respond to temporary pavement markings that are often in competition with preexisting stripes and/or misleading informal cues provided by variation in the surface characteristics of the road, shoulder, or median.
These diminished capabilities must be considered in relation to specific information needs, when negotiating work zones, while also taking into account the time (distance) in which these needs must be satisfied. The information needs may be loosely contrasted according to the discrimination of continuous versus discrete roadway features, i.e., the perception and recognition of the boundaries of the traveled way, as opposed to a singular location which must be avoided (e.g., an island, barrier, or abutment) or where a path selection decision must be acted upon (e.g., a ramp gore, pavement width transition point, or intersection). Furthermore, delineation must provide information to a driver permitting roadway feature recognition both at "long" preview distances up to and sometimes exceeding 5 s travel time, and at the more immediate proximities (within 1 s travel time) where attention is directed for instant-to-instant vehicle control responses.
An investigation of age-related differences in the required contrast for pavement delineation showed that an older driver (ages 6580) test sample required a level of contrast 2030 percent higher than a young/middle-aged (ages 1949) comparison group (Staplin et al., 1990). The differences became exaggerated with glare as an independent variable. An inevitable consequence of these age differences is an increased reliance on delineation elements for path guidance by older drivers under nighttime conditions, especially against oncoming glare. The "long preview" as well as the instant-to-instant steering control cues provided by pavement markings are critical to older drivers under these circumstances.
Raised pavement markers (RPM's) used for delineation of the centerline and edgelines in construction zones have been found to provide improved wet weather and nighttime reflectivity, and are particularly useful when lanes are diverted from their original path (Spencer, 1978). Davis (1983) reported that, compared with paint, day-night/wet-night visible RPM's improved construction zone traffic performance significantly. In this study, the markers were associated with decreased lane-change frequency and night lane encroachments. In before-and-after comparisons of accident frequencies in two construction projects, the number of accidents and fatalities decreased as a function of RPM installation (Niessner, 1978). In a study investigating vehicle guidance through work zones, Shepard (1989) recommended that closely spaced RPM's should be used as a supplement to existing pavement striping in areas where the roadway alignment changes.
Dudek, Huchingson, and Woods (1986) conducted a study on a test track to examine the effectiveness of temporary pavement markings for use in work zones. Ten candidate treatments were tested during the day, and the most effective treatments were examined at night. All treatments were tested only under dry weather/dry road conditions. The candidate treatments are presented in table 40 and included patterns with stripes, RPM's, and combinations of stripes and RPM's. Treatment 1 was the control condition in the study.
Table 40. Temporary pavement marking treatments evaluated by Dudek, Huchingson, and Woods (1986).
Results of both daylight and nighttime testing indicated that there were no practical differences between treatments when comparing measures of effectiveness developed from speed and distance measurements. Practical differences were arbitrarily defined as at least 6.5 km/h (4 mi/h) for speed measures and 0.3 m (1 ft) for distance measures. The greatest number of erratic maneuvers during daylight occurred for treatments 7 and 8, which consisted of 0.6-m (2-ft) stripes and long gaps. Drivers referred to 0.6-m (2-ft) stripes as dots. The subjective data indicated that treatments 5, 6, and 9 were preferred, under both daylight and nighttime conditions. Reasons given were that RPM's clearly identify curves, are highly visible at a great distance, provide noise and vibration when drivers cross them, and stand out more than tape markings. Of the treatments without RPM's, treatment 3 was the drivers' choice, for both lighting conditions, while treatment 2 was rated as least effective.
Because subjects tend to perform best when in a proving ground setting and because the setting is not always sensitive enough to discern small differences between candidate treatments, Dudek, Huchingson, Creasey, and Pendleton (1988) conducted field studies to compare the safety and operational effectiveness of 0.3-m (1-ft), 0.6-m (2-ft), and 1.2-m (4-ft) temporary broken line pavement markings on 12.2-m (40-ft) centers in work zones. The study was conducted at night on rural two-lane, two-way highways with 2.0-degree horizontal curvatures, level to rolling terrain, and average speeds between 80.5 km/h (50 mi/h) and 99.8 km/h (62 mi/h). In terms of speed, lateral distance, encroachment, erratic maneuver, and speed profile data for the sample of vehicles with headways of 4 s or more, there were no differences in driver performance between the 0.3-m (1-ft), 0.6-m (2-ft), and 1.2-m (4-ft) striping patterns. Analysis of subjective evaluations of the effectiveness of the markings found that the 0.3-m (1-ft) stripe was generally rated as poorest, but its mean ranking was not significantly different from that of the 0.6-m (2-ft) and 1.2-m (4-ft) stripes. Drivers generally preferred the longer stripes, but there was no evidence that the 0.6-m (2 ft) or 1.2-m (4-ft) stripes were superior to the 0.3-m (1-ft) stripe. In a discussion of the conditions present during this research, Ward (1988) stated that all sites had 3.7-m (12-ft) lanes with 1.2-m (4-ft) to 3-m (10-ft) shoulders, the marking material was highly retroreflective yellow tape laid over very black new pavement overlays, and there were no edgelines; therefore, the drivers' focus was a "brilliant ribbon of yellow to follow," resulting in no difference in driver performance between the three stripe lengths. Most important was that none of the treatments were judged as extremely effective, although the 0.3-m (1-ft) stripe was rated as poorest, and there was a slight preference for the 1.2-m (4-ft) lengths. This is consistent with results obtained by Dudek et al. (1986), where subjects rated 2.4-m (8-ft) stripes with 9.8-m (32-ft) gaps as the best striping treatment (when RPM's were not available). In the Dudek et al. (1986) study, drivers preferred the treatments with longer stripes, shorter gaps, and RPM's. Hence, the results of the Dudek et al. (1988) study may be applicable only to pavement overlay projects on two-lane, two-way rural roadways, and may not translate to other highway work-zone situations.
Harkey, Mera, and Byington (1992) conducted a study to determine the effects of short-term (interim) pavement markings on driver performance under day, night, wet, and dry weather conditions. The three marking patterns tested included: (1) 0.6-m (2-ft) stripes with 11.6-m (38-ft) gaps and no edgelines; (2) 1.2-m (4-ft) stripes with 11-m (36-ft) gaps and no edgelines; and (3) 3-m (10-ft) stripes with 9.2-m (30-ft) gaps and edgelines. The measures of effectiveness included lateral placement of the vehicle on the roadway, vehicle speed, number of edgeline and lane line encroachments, and number of erratic maneuvers (e.g., sudden speed or directional changes and brake applications). For each operational measure, the 3-m (10-ft) markings resulted in better driver performance than either the 0.6-m (2-ft) or 1.2-m (4-ft) temporary marking patterns. Drivers traveled 1.2 km/h (0.76 mi/h) slower on segments with 1.2-m (4-ft) markings and 3.3 km/h (2.02 mi/h) slower on segments marked with 0.6-m (2-ft) markings than on segments marked with 3-m (10-ft) stripes and edgelines. In addition, compared with the 3-m (10-ft) pattern, drivers encroached over the lane or edgeline 66 percent more in the presence of the 1.2-m (4-ft) temporary marking and 139 percent more in the presence of the 0.6-m (2-ft) markings. These values increased dramatically under night and wet weather conditions. Comparisons of driver performance between the 1.2-m (4-ft) and 0.6-m (2-ft) markings showed the following: (1) the speed at which drivers traveled decreased as the length of the lane line decreased; (2) drivers positioned their vehicles closer to the center of the lane as the length of the line increased; (3) the variability of vehicle placement within the lane increased as the length of the lane line decreased; (4) the number of encroachments increased as the length of the lane line decreased; and (5) all operational measures were negatively affected by adverse weather conditions. Results provided evidence of significant decreases in driver performance associated with both the 0.6-m (2-ft) and the 1.2-m (4-ft) markings, but drivers performed better with the 1.2-m (4-ft) stripes compared to the 0.6-m (2-ft) stripes. The results suggested that while it may not be practical to place full markings (3-m [10-ft]) segments with 9.2-m [30-ft] gaps as specified by MUTCD part 3A-6) on a temporary basis, measures should be taken to prevent reductions in driver performance which result in increased accident potential; such measures include the use of longer temporary markings and the appropriate use of warning signs to indicate a change in the pavement marking pattern.
Topics: research, safety, human factors, older road user, design
Keywords: research, safety, highway design, highway operations, driver age, driver performance, human factors, vision, attention, perception, cognition, memory, physical ability, risk perception, hazard perception
TRT Terms: Roads--United States--Design and construction, Older automobile drivers, Roads--United States--Visibility, Roads--United States--Safety measures, Traffic signs and signals--United States, Highway safety, Highway design, Aged drivers, Human factors