Table of Contents | List of Figures | List of Tables | Abbreviations & Acronyms | How to Use this Handbook
Recommendations:I.Intersections (At-Grade) | II. Interchanges (Grade Separation)
III. Roadway Curvature and Passing Zones | IV. Construction/Work Zones | V. Highway-Rail Grade Crossings (Passive)
Rationale and Supporting Evidence: I. Intersections (At-Grade) | II. Interchanges (Grade Separation) | III. Roadway Curvature and Passing Zones | IV. Construction/Work Zones | V. Highway-Rail Grade Crossings (Passive)
Glossary | References | Index

RATIONALE AND SUPPORTING EVIDENCE

This section of the Handbook is organized in terms of the same classes of highway features as the Recommendations: I. Intersections (At-Grade), II. Interchanges (Grade Separation), III. Roadway Curvature and Passing Zones, IV. Construction/Work Zones, and V. Highway-Rail Grade Crossings (Passive). Within each of these five classes, subsections are organized in terms of design elements with unique geometric, operational, and/or traffic control characteristics, also consistent with the recommendations.

At the beginning of each subsection within a class of highway features, reference material for a particular design element is introduced using a cross-reference table. This table relates the discussion in that subsection--as well as the associated recommendations, presented earlier--to entries in standard reference manuals consulted by practitioners in this area. Principal among these reference manuals are the Manual on Uniform Traffic Control Devices (Federal Highway Administration [FHWA], 2000); the Policy on Geometric Design of Highways and Streets [the Green Book] (American Association of State Highway and Transportation Officials [AASHTO], 1994); and the Traffic Engineering Handbook (ITE, 1999). Other standard references with more restricted applicability, which also appear in the cross-reference tables for selected design elements, include the National Cooperative Highway Research Program (NCHRP) Report No. 279, Intersection Channelization Design Guide (Neuman, 1985); Roundabouts: An Informational Guide (FHWA, 2000); the Roadway Lighting Handbook (FHWA, 1978); the Railroad-Highway Grade Crossing Handbook (FHWA, 1986); and the Highway Capacity Manual (TRB, 1998).

Material in this part of the Handbook represents, to as great an extent as possible at the time of its development, the results of empirical work with older driver or pedestrian samples for investigations with the specific highway features of interest. Observational and controlled field studies were given precedence, together with laboratory simulations employing traffic stimuli and relevant situational cues. Crash data are cited as appropriate. In addition, some citations reference studies showing effects of design changes, where the predicted impact on (older) driver performance is tied logically to the results of research on age differences in response capability.

I. INTERSECTIONS (AT-GRADE)

The following discussion presents the rationale and supporting evidence for Handbook recommendations pertaining to these 17 design elements (A-Q):

A. Design Element: Intersecting Angle (Skew)

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

There is broad agreement that right-angle intersections are the preferred design. Decreasing the angle of the intersection makes detection of and judgments about potential conflicting vehicles on crossing roadways much more difficult. In addition, the amount of time required to maneuver through the intersection increases, for both vehicles and pedestrians, due to the increased pavement area. However, there is some inconsistency among reference sources concerning the degree of skew that can be safely designed into an intersection. The Green Book states that although a right-angle crossing normally is desired, an angle of 60 degrees provides most of the benefits that are obtained with a right-angle intersection. Subsequently, factors to adjust intersection sight distances for skewness are suggested for use only when angles are less than 60 degrees (AASHTO, 1994). However, another source on subdivision street design states that: "Skewed intersections should be avoided, and in no case should the angle be less than 75 degrees" (Institute of Transportation Engineers [ITE], 1984). The Traffic Engineering Handbook (ITE, 1999) states that: "Crossing roadways should intersect at 90 degrees if possible, and not less than 75 degrees." It further states that: "Intersections with severe skew angles (e.g., 60 degrees or less) often experience operational or safety problems. Reconstruction of such locations or institution of more positive traffic control such as signalization is often necessary." With regard to intersection design issues on two-lane rural highways, ITE (1999) states that: "Skew angles in excess of 75 degrees often create special problems at stop-controlled rural intersections. The angle complicates the vision triangle for the stopped vehicle; increases the time to cross the through road; and results in a larger, more potentially confusing intersection."

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

Isler, Parsonson, and Hansson (1997) measured the maximum head rotation of 20 drivers in each of four age groups: less than age 30; ages 40 to 59; ages 60 to 69; and age 70 and older, as well as their horizontal peripheral visual field. The oldest subjects exhibited an average decrement of approximately one-third of head range of movement compared with the youngest group of subjects. The mean maximum head movement (in one direction) was 86 degrees for the youngest drivers, 72 degrees for drivers ages 40 to 59, 67 degrees for drivers ages 60 to 69, and 59 degrees for drivers age 70+. In addition, the percentage of drivers with less than 30 degrees of horizontal peripheral vision increased with increases in age, from 15 percent of the younger driver sample to 65 percent of the drivers age 70+. Three of the oldest drivers had less than 50 degrees of head movement and two of these drivers also had less than 20 degrees of horizontal peripheral vision.

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

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

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

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

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

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

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

Lane widths are addressed in the Intersection Channelization Design Guide (Neuman, 1985). A recommendation for (left) turning lanes, which also applies to receiving lanes, is that "3.6-m (12-ft) widths are desirable, (although) lesser widths may function effectively and safely. Absolute minimum widths of 2.7 m (9 ft) should be used only in unusual circumstances, and only on low-speed streets with minor truck volumes." Similarly, the ITE (1984) guidelines suggest a minimum lane width of 3.3 m (11 ft) and specify 3.6 m (12 ft) as desirable. These guidelines suggest that wider lanes be avoided due to the resulting increase in pedestrian crossing distances. However, the ITE guidelines provide a range of lane widths at intersections from 2.7 m to 4.3 m (9 ft to 14 ft), where the wider lanes would be used to accommodate larger turning vehicles, which have turning paths that sweep a path from 4.1 m (13.6 ft) for a single-unit truck or bus, up to 6.3 m (20.6 ft) for a semitrailer. Thus, wider (3.6 m [12 ft]) lanes used to accommodate (right) turning trucks also are expected to benefit (left) turning drivers. Further increases in lane width for accommodation of heavy vehicles may result in unacceptable increases in (older) pedestrian crossing times, however.

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

Figure 1. Turning path taken by left-turning vehicles, where 1=encroach into opposing cross-traffic stream; 2, 3, and 4=proper turning from different points within the intersection; and 5=left turn from a position requiring a greater-than-90-degree turn to enter the cross street.

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

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

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

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

The effectiveness of channelization from a safety perspective has been documented in several studies. An evaluation of Highway Safety Improvement Program projects showed that channelization produced an average benefit-cost ratio of 4.5 (FHWA, 1996). In this evaluation, roadway improvements consisting of turning lanes and traffic channelization resulted in a 47 percent reduction in fatal crashes, a 26 percent reduction in nonfatal injury crashes, and a 27 percent reduction in combined fatal plus nonfatal injury crashes, at locations where before and after exposure data were available.

One of the advantages of using curbed medians and intersection channelization is that it provides a better indication to motorists of the proper use of travel lanes at intersections. In a set of studies performed by the California Department of Public Works investigating the differences in crash experience with raised channelization versus channelization accomplished through the use of flush pavement markings, the findings were as follows: raised traffic islands are more effective than flush marked islands in reducing frequencies of night crashes, particularly in urban areas; and little difference is noted in the effectiveness of raised versus marked channelizing islands at rural intersections (Neuman, 1985).

One of the most common uses of channelization is for the separation of left-turning vehicles from the through-traffic stream. The safety benefits of left-turn channelization have been documented in several studies. A study by McFarland, Griffin, Rollins, Stockton, Phillips, and Dudek (1979) showed that crashes at signalized intersections where a left-turn lane was added, in combination with and without a left-turn signal phase, were reduced by 36 percent and 15 percent, respectively. At nonsignalized intersections with marked channelization separating the left-turn lane from the through lane, crashes were reduced for rural, suburban, and urban areas by 50, 30, and 15 percent, respectively. When raised channelization devices were used, the crash reductions were 60, 65, and 70 percent in rural, suburban, and urban areas, respectively. Consistent findings were reported in Hagenauer, Upchurch, Warren, and Rosenbaum (1982).

Important considerations in choosing to implement raised versus marked channelization include operating speed and type of maneuver (i.e., left turn versus right turn). Left-turn channelization separating through and turning lanes may, because of its placement, constitute a hazard when a raised treatment is applied, especially on high-speed facilities. Detection and avoidance of such hazards requires visual and response capabilities known to decline significantly with advancing age.

Another benefit in the use of channelization is the provision of a refuge for pedestrians. Refuge islands are a design element that can aid older pedestrians who have slow walking speeds. With respect to the Hagenauer et al. (1982) study cited earlier, Hauer (1988) stated that because channelization in general serves to simplify an otherwise ambiguous and complex situation, the channelization of an existing intersection might enhance both the safety and mobility of older persons, as well as enhance the safety of other pedestrians and drivers. However, in designing a new intersection, he stated that the presence of islands is unlikely to offset the disadvantage of large intersection size for the pedestrian.

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

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

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

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

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

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

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

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

Regarding channelization for mid-block left-turn treatments. Bonneson and McCoy (1997) evaluated the safety and operational effects of three mid-block left-turn treatments: raised curb medians; two-way, left turn-lanes; and undivided cross sections. Traffic flow data were collected during 32 field studies in 8 cities in 4 States, and 3-year crash histories for 189 street segments were obtained from cities in 2 States. The studies were conducted on urban or suburban arterial segments, and therefore recommendations can only be applied to such environments that include the following criteria: traffic volume exceeding 7,000 vehicles per day; speed limit between 48 and 80 km/h (30 and 50 mi/h); spacing of at least 107 m (350 ft) between signalized intersections; direct access from abutting properties; no angle curb parking (parallel parking is acceptable); located in or near a populated area (e.g., population of 20,000 or more); no more than six through lanes (three in each direction); and arterial length of at least 1.2 km (0.75 mi.).

In terms of annual delays to major-street left-turn and through vehicles, the raised-curb treatment has slightly higher delays than the TWLTL treatment at the highest left-turn and through volumes, which results from the greater likelihood of bay overflow for the raised-curb median treatment under high-volume conditions. The undivided cross section has significantly higher delays than the raised-curb treatment for all nonzero combinations of left-turn and through volume.

Looking at crash frequencies as a function of mid-block channelization treatment, the raised curb median treatment is associated with the fewest crashes of all three treatment types. Differences between the crash frequencies for TWLTL treatments vs undivided cross sections are affected by whether or not parallel parking is allowed on the undivided cross section. When parallel parking is allowed on the undivided cross section, the undivided cross section is associated with significantly more crashes than the TWLTL treatment. However, when parallel parking is not allowed, the TWLTL has about the same crash frequency as the undivided cross section at lower traffic volumes.

In general, at mid-block locations, the raised-curb median treatment was associated with fewer crashes than the undivided cross section and TWLTL, especially for average daily traffic demands greater than 20,000 vehicles per day. Also, a benefit of the raised-curb median is that it provides a pedestrian refuge.

Bonneson and McCoy (1997) provide a set of six tables to use as guidelines in considering the conversion of an undivided cross section to a raised curb median, or to a TWLTL, and conversion from a TWLTL to a raised-curb median treatment. In these tables, it is recommended that the existing treatment remain in place when the benefit-cost ratio (in terms of delay and safety) is less than 1.0, and when the benefit-cost ratio exceeds 2.0, it is recommended that the engineer consider adding the alternative treatment.

Bonneson and McCoy (1997) do not report crash frequencies by driver age, for one treatment versus another. However, approximately one-fifth of the older drivers participating in focus group studies conducted by Staplin, Harkey, Lococo, and Tarawneh (1997) reported that using center two-way left turn lanes (TWLTL), was confusing, risky, and made them uncomfortable, because at times they have come face-to-face with an opposing left turner, and both drivers were stranded. Also mentioned was the difficulty seeing the pavement markings in poor weather (night, fog, rain) when they are less visible, and particularly when they are snow covered. Drivers referred to TWLTL's as "suicide lanes." In the same research study, Staplin et al. (1997) reported on a crash analysis that revealed ways in which older drivers failed to use a TWLTL correctly: a TWLTL was not used for turning at all; and the TWLTL was entered too far in advance of where the turn was to be made.

D. Design Element: Intersection Sight Distance Requirements

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

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

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

Harwood, Mason, Pietrucha, Brydia, Hostetter, and Gittings (1993) stated that the provision of intersection sight distance (ISD) is intended to give drivers an opportunity to obtain the information they need to make decisions about whether to proceed, slow, or stop in situations where potentially conflicting vehicles may be present. They noted that while it is desirable to provide a reasonable margin of safety to accommodate incorrect or delayed driver decisions, there are substantial costs associated with providing sight distances at intersections; therefore, it is important to understand the derivation of ISD requirements and why it is reasonable to expect a safety benefit from tailoring this design parameter to the needs of older drivers.

Traditionally, the need for--as well as the basis for calculating--sight distances at intersections has rested upon the notion of the sight triangle. This is diagrammed in figure 3. As excerpted from NCHRP Report 383, this diagram effectively illustrates how different driver decisions during a (minor) road approach to an intersection (with a major road) depend upon the planned action. The driver's first decision is to either stop or to continue through the intersection (with a turning or a crossing maneuver) according to the type of traffic control information he or she perceives. A red signal or a stop sign results in a "stop" decision; all other types of information are functionally equivalent at this stage of driver decision making, translating into a "yield" decision. That is, drivers' decisions at this stage are dichotomous: (1) slow down and prepare to stop, regardless of traffic on the major road, or (2) based on their view of the major road, either slow down, maintain speed, or accelerate as required to safely complete their intended maneuver. For drivers who are required to stop, their decision to proceed after the stop also is based

Figure 3. Sight distance for left and right turns for passenger car drivers at yield-controlled intersections. Source: Harwood et al. (1993).

on a view of traffic on the major road, but at a point much closer to the intersection. The contrasting sight lines and sight triangles defined by the position of a driver who must stop before proceeding at the intersection, versus one who may proceed without stopping, conditional on the intersecting (major) road traffic, are clearly indicated in figure 3.

For purposes of describing driver decision making, the diagram in figure 3 may apply to varying aspects of intersection operations in all Cases I through IV as per current AASHTO classification. For Case V, however, where a driver is turning left from a major road at an intersection or driveway, the decision process and corresponding sight distance requirements are defined differently. The sight lines in this case are defined by the presence, type (passenger versus heavy vehicle), and location (positioned or unpositioned in the intersection) of opposing left-turning traffic, and by the lateral offset of the opposite left-turn lanes themselves. These relationships are illustrated in figure 4 from McCoy, Navarro, and Witt (1992).

Figure 4. Spatial relationships that determine available sight distance. Source: McCoy, Navarro, and Witt (1992).

It is also important to acknowledge recent thinking (cf. Harwood et al. 1993) that bases recommended sight distances upon the observed gaps that drivers will accept for performing various maneuvers at intersections--specifically, upon the "critical gap." This is the distance, expressed in the number of seconds (at operating speed) of separation between a subject vehicle and a conflict vehicle, where the driver of the subject vehicle will make a decision to proceed with a maneuver ahead of the conflict vehicle 50 percent of the time. The "Gap Acceptance" model yields different, typically shorter ISD requirements, than the existing (or modified) AASHTO model. It also classifies intersection operations in a somewhat different manner than AASHTO (1994).

Both the assumptions underlying the current AASHTO (1994) model and the Gap Acceptance model, respectively, received consideration in developing the Handbook's recommendations for this design element. Apart from the theoretical differences between these models, there is also the practical matter that it may take some time for designers and engineers who are familiar with and have worked successfully with one approach to embrace an alternative approach. Thus, this Handbook seeks to accommodate the full range of design practices, to the extent that data provide an understanding of the intersection sight distance requirements of older drivers. Where ISD requirements are defined through application of formulas incorporating "perception-reaction time" (PRT), the broad and well-documented age differences in this aspect of driver performance support recommendations for all included cases (I through V). Where ISD requirements are determined through a formula that depends upon gap size, however, recommendations must be limited at this time to cases where gap acceptance by older versus younger drivers has been empirically studied (Case F).

The rationale for recommendations pertaining to intersection sight distance requirements will proceed as follows. First, driver age differences in cognitive and physical capabilities that are relevant to ISD issues will be discussed. Then, research efforts that have attempted to quantify the safety impact of providing adequate sight distance are summarized, plus studies examining the appropriate values for specific components used when calculating sight distance in the AASHTO and Gap Acceptance models.

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

older persons have greater difficulty in situations where planned actions must be rapidly altered (Stelmach, Goggin, and Amrhein, 1988). The difficulty older persons experience in making extensive and repeated head movements further increases the decision and response times of older drivers at intersections.

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

Other studies have attempted to show the benefits to be gained from improvements to ISD (Mitchell, 1972; Strate, 1980). Mitchell conducted a before-and-after analysis, with a period of 1 year on each end, of intersections where a variety of improvements were implemented. The results showed a 67 percent reduction (from 39 to 13) in crashes where obstructions that inhibited sight distance were removed; this was the most effective of the implemented improvements. Strate's analysis examined 34 types of improvements made in Federal Highway Safety Program projects. The results indicated that sight distance improvements were the most cost-effective, producing a benefit-cost ratio of 5.33:1. The more recent report on the FHWA Highway Safety Improvement Programs (1996) indicates that improvements in intersection sight distance have a benefit-cost ratio of 6.1 in reducing fatal and injury crashes. In these analyses, fatal crashes were reduced by 56 percent and nonfatal injury crashes by 37 percent after sight distance improvements were implemented.

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

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

By comparison, for applications of the Gap Acceptance model, an alternative classification system has been proposed (Harwood et al., 1996):

• Case A: Intersections with no control.
• Case B: Intersections with Stop control on the minor road.

  Case B1: Left turn from the minor road.
  Case B2: Right turn from the minor road.
  Case B3: Crossing maneuver from the minor road

• Case C: Intersections with Yield control on the minor road.
  

  Case C1: Crossing maneuver from the minor road.
  Case C2: Left or right turn from the minor road.

• Case D: Intersections with traffic signal control.


• Case E: Intersections with all-way Stop control.


• Case F: Intersections where a stopped vehicle is turning left from a major road.

One of the principal components in determining ISD in all cases defined according to AASHTO (1994) is perception-reaction time (PRT). The discussion of this value is first presented in chapters 2 and 3 of the Green Book under "Reaction Time" and "Brake Reaction Time," respectively (AASHTO, 1994). Results of several studies (e.g., Normann, 1953; Johansson and Rumar, 1971) are cited, and in conclusion, the 2.5-s value is selected since it was found to be adequate for approximately 90 percent of the overall driver population. Controlled field studies and simulator studies involving older drivers have confirmed that brake reaction times to unexpected hazards (e.g., a barrel rolling into the road in front of the driver; a vehicle turning in front of a driver who is traveling straight through an intersection) are not significantly different as a function of age, and that virtually all response times are captured by the current 2.5-s AASHTO design parameter for brake perception-response time (Lerner, Huey, McGee, and Sullivan, 1995; Kloeppel, Peters, James, Fox, and Alicandri, 1995).

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

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

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

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

Fambro, Koppa, Picha, and Fitzpatrick (1998) found significant differences in mean perception-brake response times as a function of age and gender, with older drivers and female drivers demonstrating longer response times. They conducted three separate on-road studies to measure driver perception-brake response time to several stopping sight distance situations. Studies were conducted on a closed course as well as on an open roadway. In one study conducted on the closed course, subjects drove an instrumented test vehicle belonging to the Texas Transportation Institute, and in another closed course study they drove their own vehicles. In the open roadway study, they drove their own vehicles. Seventeen younger drivers (age 24 or under) and 21 older drivers (age 55 or older) participated in trials that required them to brake in response to expected and unexpected events, that included a barrel rolling off of a pick up truck parked next to the roadway, an illuminated LED on the windshield, and a horizontal blockade that deployed ahead of them on the roadway. Across all expected-object, perception-brake response time trials, the mean response time for younger drivers was 0.52 s and the mean response time for older drivers was 0.66 s. For these "expected" trials, the mean perception brake-response time for males was 0.59 s and for females was 0.63 s. For the unexpected-object, perception-brake response trials, longer response times were demonstrated for trials where subjects drove their own vehicles, compared to those in which they drove the Transportation Institute's vehicle. The study authors suggested that subjects were more relaxed and unsuspecting when driving their own vehicles. The mean response time across studies (controlled and open road, own vehicle and research vehicle) for the unexpected object was 1.1 s; the 95^th percentile perception-brake response time was 2.0 s.

Based on this finding, Fambro et al. (1998) concluded that AASHTO's 2.5-s perception-brake reaction time value is appropriate for highway design, when stopping sight distance is the relevant control. However, they note that at locations or for geometric features where something other than stopping sight distance is the relevant control, different perception-reaction times may be appropriate. For example, longer perception-reaction times may be appropriate for intersection or interchange design where more complex decisions and driver speed and/or path correction are required.

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

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

In contrast, recent research conducted by Lerner, Huey, McGee, and Sullivan (1995) concluded that, based on older driver performance, no changes to design PRT values were recommended for ISD, SSD, or decision sight distance (DSD), even though the 85th percentile J values exceeded the AASHTO 2.0-s design standard at 7 of the 14 sites. The J value equals the sum of the PRT time and the time to set the vehicle in motion, in seconds. No change was recommended because the experimental design represented a worst-case scenario for visual search and detection (drivers were required to begin their search only after they had stopped at the intersection and looked inside the vehicle to perform a secondary task). Naylor and Graham (1997), in a field study of older and younger drivers waiting to turn left at stop-controlled intersections (Case IIIB), similarly concluded that the current AASHTO value of 2.0 s is adequate for the PRT (J-value) used in calculating intersection sight distance at these sites.

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

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

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

ISD=1.47 V (J +t_a) English

ISD=0.278 V (J +t_a) Metric

where:

ISD = intersection sight distance (feet for English equation; meters for metric equation).

V = major roadway operating speed (mi/h for English equation; km/h for metric equation).

J = time required to search for oncoming vehicles, to perceive that there is sufficient time to make the left turn, and to shift gears, if necessary, prior to starting (J is currently assumed to be 2.0 s).

t_a = time required to accelerate and traverse the distance to clear traffic in the approaching lane(s); obtained from figure IX-33 in the AASHTO Green Book.

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

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

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

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

ISD = 1.47 VG English

ISD = 0.278 VG Metric

where:

ISD = intersection sight distance (feet for English equation; meters for metric equation).

V = operating speed on the major road (mi/h for English equation, km/h for metric equation).

G = the specified critical gap (in seconds); equal to 5.5 s for crossing one opposing lane plus an additional 0.5 s for each additional opposing lane.

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

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

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

Staplin, Harkey, Lococo, and Tarawneh (1997) conducted an observational field study of driver performance as a function of left-turn lane geometry and driver age, at four locations where the main road operating speed was 56 km/h (35 mi/h). The mean left-turn critical gap sizes (in seconds) across all sites, for drivers who had positioned their vehicles within the intersection, were as follows: 5.90 s for the young/middle-aged (ages 25-45) females; 5.91 s for the young/middle-aged males; 6.01 s for the young-old (ages 65-74) females; 5.84 s for the young-old males; 6.71 s for the old-old (age 75 and older) females; and 6.55 s for the old-old males. Prominent trends indicated that older drivers demonstrated larger critical gap values at all locations. The young/middle-aged and young-old groups were not significantly different from each other; however, both were significantly different from the old-old group. Critical gap data were not collected in this study for drivers who did not position themselves within the intersection, but it is important to note that the older drivers were less likely to position themselves within the intersection than the young and middle-aged drivers.

Critical gap sizes displayed in a laboratory simulation study in the same project, where oncoming vehicles traveling at 56 km/h (35 mi/h) were viewed on a large screen display in correct perspective, ranged from 6.4 s to 8.1 s for young/middle-aged drivers and from 5.8 to 10.0 s for drivers age 75 and older. This increase in size and variability of the critical gap for left turns by older drivers suggests that the value for G in the Gap Acceptance model must be increased to accommodate this user group, beyond levels recommended in NCHRP 383 (where the performance of older drivers, per se, was not at issue).

The culmination of this work was a rigorous exercise of competing models and theoretical approaches for calculating sight distance requirements. As reported by Staplin et al. (1997) current and proposed sight distance models were exercised using data collected in the observational field study. This study was conducted at four intersections which differed in the amount that the opposite left-turn lanes were offset The goal was to determine which model(s), including existing and modified AASHTO models and a Gap Acceptance model, best predicted the data observed in the field.

Several data elements collected in the field received special attention. One of these data elements was the maneuver time of the left-turning driver. This time is equivalent to t_a in the AASHTO model, reference figure IX-33 in the AASHTO (1994) Green Book. These times were measured at each of four intersections included in the study, for positioned and unpositioned drivers. That is, separate maneuver-time measures were obtained, depending on whether or not the drivers positioned themselves within the intersection prior to turning. Staplin et al. (1997) found no significant differences in maneuver time as a function of age for the drivers turning left at the four intersections studied (which had distances ranging from 26 to 32 m [84 to 106 ft]). Maneuver times for drivers positioned within the intersection versus unpositioned drivers, however, were significantly different. Since older drivers less frequently positioned themselves in the field study, the design value for this factor (maneuver time) should be based on that obtained for unpositioned drivers.

A comparison between AASHTO values and the 95th percentile clearance times demonstrated by positioned drivers and unpositioned drivers in this study is presented in table 6. In table 6, the "positioned" vehicles were located within the intersection, approaching the median or centerline of the cross street. The "unpositioned" vehicles were at or behind the stop line or end of the left-turn bay. (See figure 8 located in the discussion for Design Element E, for an illustration of driver positioning within an intersection).

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

A detailed discussion of the outputs from the model exercise is provided in the publication Intersection Geometric Design and Operational Guidelines for Older Drivers and Pedestrians (Staplin, Harkey, Lococo, and Tarawneh, 1997). However, the most significant result for purposes of this discussion is as follows: the required sight distances computed using a modified AASHTO model (where PRT was increased to 2.5 s) produced values that were most predictive of actual field operations.

Thus, when ISD is calculated using the AASHTO model as it relates to drivers turning left from a major roadway, there is evidence that the PRT value should be increased to 2.5 s to provide adequate sight distance. The Gap Acceptance model, on the other hand, produced sight distance values that were approximately 23 percent shorter than the current AASHTO model,

1 ft = 0.305 ft. 1 mi/h = 1.61 km/h Figure 5. Comparison of sight intersection distance values calulated using modified AASHTO model (J=2.5 s) and Gap Acceptance model using varying values for G.

that uses a PRT of only 2.0 s. If the Gap Acceptance model is going to be used, particularly where there are significant volumes of older left-turning drivers, an adjustment factor applied to increase the sight distance to better accommodate this driver age group therefore appears warranted.

To determine what adjustment is most appropriate in this regard, a set of analyses were performed in which the goal was to identify a value of G that would yield required sight distance values meeting or exceeding those derived from the modified AASHTO model formula (i.e., where J = 2.5 s). By extension, this result would also best match the behavior of drivers 75 and older observed in the field study.

Very simply, alternate values for G were substituted into the gap formula for calculating minimum required sight distance (1.47VG). These included 5.5 s, as recommended by Harwood et al.(1996), as well values which increase in 0.5 s increments. The results of these calculations for alternate values of G, beginning at 7.0 s, are plotted against the required sight distance calculated using the modified AASHTO formula [1.47V(J+t_a); where J=2.5 s and t_a is obtained from table IX-33 in the Green Book] in figure 5. As shown in this figure, a gap of 8.0 s affords sight distance for left-turning drivers that equals or exceeds the requirements calculated using the modified AASHTO model for major road design speeds from 32 km/h to 113 km/h (20 mi/h to 70 mi/h).

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

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

Studies examining older driver crashes and the types of maneuvers being performed just prior to the collision have consistently found this group to be overinvolved in left-turning crashes at both rural and urban signalized intersections and have indicated that failure to yield the right-of-way (as the turning driver) was the principal violation type (Staplin and Lyles, 1991; Council and Zegeer, 1992). Underlying problems for the maneuver errors include the misjudgment of oncoming vehicle speed, misjudgment of available gap, assuming the oncoming vehicle was going to stop or turn, and simply not seeing the other vehicle. Joshua and Saka (1992) noted that sight distance problems at intersections which result from queued vehicles in opposite left-turn lanes pose safety and capacity deficiencies, particularly for unprotected (permitted) left-turn movements. These researchers found a strong correlation between the offset for opposite left-turn lanes--i.e., the distance from the inner edge of a left-turn lane to the outer edge of the opposite left-turn lane--and the available sight distance for left-turning traffic.

The alignment of opposite left-turn lanes and the horizontal and vertical curvature on the approaches are the principal geometric design elements that determine how much sight distance is available to a left-turning driver. Operationally, vehicles in the opposite left-turn lane waiting to turn left can also restrict the (left-turning) driver's view of oncoming traffic in the through lanes. The level of blockage depends on how the opposite left-turn lanes are aligned with respect to each other, as well as the type/size of vehicles in the opposing queue. Restricted sight distance can be minimized or eliminated by offsetting opposite left-turn lanes so that left-turning drivers do not block each other's view of oncoming through traffic. When the two left-turn lanes are exactly aligned, the offset distance has a value of zero. Negative offset describes the situation where the opposite left-turn lane is shifted to the left. Positive offset describes the situation where the opposite left-turn lane is shifted to the right. Figure 6 illustrates the relationships between the opposite left-turn lanes for negative and positive offset lane geometry. Positive offset left-turn lanes and aligned left-turn lanes provide greater sight distances than negative offset left-turn lanes, and a positive offset provides greater sight distance than the aligned configuration.

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

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

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

Figure 7 compares conventional left-turn lanes with these two alternative designs. As noted by Harwood et al. (1995), parallel and tapered offset left-turn lanes are still not common, but are used increasingly to reduce the risk of crashes due to sight restrictions from opposite left-turn vehicles. Parallel offset left-turn lanes with 3.6-m (12-ft) widths can be constructed in raised medians with widths as narrow as 7.2 m (24 ft), and can be provided in narrower medians if restricted lane widths or curb offsets are used or a flush median is provided (Bonneson, McCoy, and Truby, 1993). Tapered offset left-turn lanes generally require raised medians of 7.2 m (24 ft) or more in width.

For separation of the left-turn lane from through traffic in alternative designs such as those discussed above, the practitioner must choose between raised channelization and channelization accomplished through the use of pavement markings. As noted earlier, left-turn channelization separating through and turning lanes may, because of its placement, constitute a hazard when a raised treatment is applied, especially on high-speed facilities. Detection and avoidance of such hazards requires visual and response capabilities known to decline significantly with advancing age, supporting recommendations for treatments to improve the contrast for these channelizing features at intersections (see Design Element C).

Figure 7. Alternative Left-turn treatments for rural and suburban divided highways. Source: Bonneson, McCoy and Truby (1993).

As discussed in some detail under Design Element D, Staplin, Harkey, Lococo, and Tarawneh (1997) performed a laboratory study, field study, and sight distance analysis to measure driver age differences in performance under varying traffic and operating conditions, as a function of varying degrees of offset of opposite left-turn lanes at suburban arterial intersections. Research findings indicated that an increase in sight distance through positively offsetting left-turn lanes can be beneficial to left-turning drivers, particularly older drivers. In the field study, where left-turn vehicles needed to cross the paths of two or three lanes of conflicting traffic (excluding parking lanes) at 90-degree, four-legged intersections, four levels of offset of opposite left-turn lane geometry were examined. These levels include: (a) 1.8-m (6-ft) "partial positive" offset, (b) aligned (no offset) left-turn lanes, (c) 0.91-m (3-ft) "partial negative" offset, and (d) 4.3-m (14-ft) "full negative" offset. All intersections were located within a growing urban area where the posted speed limit was 56 km/h (35 mi/h). Additionally, all intersections were controlled by traffic-responsive semi-actuated signals, and all left-turn maneuvers were completed during the permitted left-turn phase at all study sites.

In the analysis of the field study lateral positioning data, it was found that the partial positive offset and aligned locations had the same effect on the lateral positioning behavior of drivers. Drivers moved approximately 1.5 m (5 ft) to the left when there was a large negative offset, clearly indicating that sight distance was limited. There was a significant difference between the partial negative offset geometry and the partial positive offset or aligned geometries, suggesting a need for longer sight distance when opposite left-turn lanes are even partially negatively offset. The fact that older drivers (and females) were less likely to position themselves (i.e., pull into the intersection) in the field studies highlights the importance ofproviding adequate sight distance for unpositioned drivers, for all left-turn designs. Vehicle positioning refers to the location within an intersection at which a left-turning vehicle waits for an acceptable gap in the opposing through traffic stream; specifically, at issue is the positioning behavior of drivers attempting to make a left turn through the conflicting through traffic while being opposed or blocked by at least one vehicle trying to make a left-turn maneuver from the opposite direction. The restriction of sight distance for an unpositioned versus a positioned driver at an intersection with aligned left-turn lanes is shown in figure 8.

Figure 8. Difference in sight-distance restriction for an unpositioned driver and a positioned driver at an aligned intersection with an opposing left-turning driver.

Several issues were raised in the research conducted by Staplin et al. (1997) regarding the adequacy of the current and proposed intersection sight distance (ISD) models for a driver turning left from a major roadway. The researchers exercised alternative sight distance models, including the current AASHTO Case V model using 2.0 s for perception-reaction time (PRT), a modified AASHTO model using a 2.5-s PRT, and a Gap Acceptance model proposed in NCHRP 383 by Harwood, Mason, Brydia, Pietrucha, and Gittings (1996). The proposed Gap Acceptance model relies on a critical gap value in place of PRT and maneuver time. A detailed description of the model parameters and output can be found in the FHWA report entitled Intersection Geometric Design and Operational Guidelines for Older Drivers and Pedestrians (Staplin et al., 1997). Of particular significance was the finding that the modified AASHTO model with the longer PRT of 2.5 s was the model most predictive of actual field operations. Also of significance was the dramatic decrease in required sight distance that occurred with the gap acceptance model compared with the traditional AASHTO model. Across all intersections and all design speeds, the required sight distance was approximately 23 percent less using the gap acceptance model. However, this was expected since the rationale behind the use of a gap acceptance model (cf. Harwood et al., 1996), in place of the current AASHTO model, is the fact that drivers are commonly observed accepting shorter gaps than those implied by the current model. As discussed under Design Element D, subsequent analyses established a recommendation for use of an 8.0-s gap size (plus 0.5 s for each additional lane crossed) to adjust the Gap Acceptance to accommodate older driver needs for increased sight distance.

Regardless of which model is used to compute ISD for drivers turning left off a major roadway, a practical countermeasure to increase the sight distance is through positive offset of left-turn lanes. As shown in the study by Staplin et al. (1997), such designs result in significantly better performance on the part of all drivers, but especially for older drivers. Prior work by McCoy et al. (1992) examined the issue of offset left-turn lanes, and developed an approach that could be used to compute the amount of offset that is required to minimize or eliminate the sight restriction caused by opposing left-turn vehicles.

This approach, incorporating the parameters represented in the intersection diagram shown earlier in figure 4 (see Design Element D), was applied to the intersections in the study by Staplin et al. (1997) to determine the amount of offset that would be required when using the modified AASHTO model (i.e., J = 2.5 s). The left-turn lane offsets required to achieve the minimum required sight distances calculated using this model are shown in figure 9, in addition to the offsets required to provide unrestricted sight distance. Based on intersections examined in the study, the offset necessary to achieve unrestricted sight distance for opposing left-turning cars is 1.2 m (4.1 ft) and for opposing left-turning trucks is 1.7 m (5.6 ft).

1 ft = 0.305 ft. 1 mi/h = 1.61 km/h Figure 9. Left-turn lane offset design values necessary to achieve unrestricted sight distances calculated using either the modified AASHTO model (J= 2.5 s) or the Gap Acceptance Model with G=8.0s.

Finally, the potential for wrong-way maneuvers, particularly by older drivers, at intersections with positive offset channelized left-turn lanes was raised during a panel meeting comprised of older driver experts and highway design engineers, during the conduct of the research performed by Staplin et al. (1997). The concern expressed was that drivers turning left from the minor road may turn too soon and enter the channelized left-turn lane, instead of turning around both medians. Similar concern was raised by highway engineers surveyed by Harwood et al. (1995) during the conduct of NCHRP project 15-14(2). These authors reported that the potential for wrong-way movements by opposing-direction vehicles entering the left-turn roadway is minimal if proper signing and pavement markings are used.

Researchers studying wrong-way movements at intersections--particularly the intersection of freeway exits with secondary roads--have found that such movements resulted from left-turning vehicles making an early left turn rather than turning around the nose of the median, and have proposed and tested several countermeasures. Scifres and Loutzenheiser (1975) reported that indistinct medians are design elements that reduce a driver's ability to see and understand the overall physical and operational features of an intersection, increasing the frequency of wrong-way movements. They suggested delineation of the median noses to increase their visibility and improve driver understanding of the intersection design and function. Also, increasing the conspicuity of ONE WAY, WRONG WAY, and DO NOT ENTER signs by using larger-than-standard (MUTCD) size signs, and using retroreflective sheeting on these signs that provides for high brightness at the wide observation angles typical of the sign placements and distances at which these signs are viewed (e.g., 1.0+ degrees) will be of benefit to drivers, particularly those with age-related diminished visual and attentional capabilities. Parsonson and Marks (1979) found that the use of the two-piece, 7.1-m- (23.5-ft-) long arrow pavement marking (wrong-way arrow) was effective in preventing wrong-way entries onto freeway exit ramps in Georgia. Later work in this State found a benefit of pulling the nose back from the intersection, and extending the median line from the nose to the intersection using painted markings and raised retroreflectors; this treatment reduced the frequency of impacts with the median by turning vehicles, particularly trucks. ⁽¹⁾

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

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

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

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

A variety of conspicuity-enhancing treatments are mandated in current practice. The MUTCD (section 3B.10, Approach Markings for Obstructions) specifies that pavement markings shall be used to guide traffic away from fixed objects (such as median islands and channelization islands) within a paved roadway. Section 3B.21 (Curb Markings) states that retroreflective solid yellow markings should be placed on the curbs of islands that are located in the line of traffic flow where the curb serves to channel traffic to the right of the obstruction, and that retroreflective solid white markings should be used (on curbs) when traffic may pass on either side of the island. Section 3E.01 (Colored Pavements) describes the use of colored pavements as traffic control devices, where yellow shall be used for median islands and white for channelizing islands, and section 3G.03 (Island Marking Application) describes the use of pavement and curb markings; object markers; and delineators for island marking application. Supplementary treatments, and requirements for in-service brightness levels for certain elements contained in existing guidelines, are presently at issue.

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

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

To describe the magnitude of the effects of age and visual ability on delineation detection/recognition distance and retroreflective requirements for threshold detection of pavement markings, a series of analyses using the Ford Motor Company PC DETECT computer model (cf. Matle and Bhise, 1984) yielded the stripe contrast requirements shown in table 9. PC DETECT is a headlamp seeing-distance model that 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