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Publication Number: FHWA-RD-97-135
Date: January 1998

Older Driver Highway Design Handbook

II. INTERCHANGES (GRADE SEPARATION)

  1. Exit Signing and Exit Ramp Gore Delineation
  2. Acceleration/Deceleration Lane Design Features
  3. Fixed Lighting Installations
  4. Traffic Control Devices for Prohibited Movements on Freeway Ramps
Background and Scope of Handbook Recommendations

Overall, freeways are characterized by the highest safety level (lowest fatality rates) when compared with other types of highways in rural and urban areas (American Automobile Association Foundation for Traffic Safety, 1995). At the same time, freeway interchanges have design features that have been shown to result in significant safety and operational problems. Taylor and McGee (1973) reported more than 20 years ago that erratic maneuvers are a common occurrence at freeway exit ramps and that the number of accidents in the vicinity of the exit ramp is four times greater than at any other freeway location. Two decades later, Lunenfeld (1993) reiterated that most freeway accidents and directional uncertainty occur in the vicinity of interchanges.

Distinct patterns in the occurrence of freeway interchange accidents emerge in studies that look specifically at driver age. Staplin and Lyles (1991) conducted a statewide (Michigan) analysis of the accident involvement ratios and types of violations for drivers in the following age groups: age 76 and older; ages 56­75, ages 27­55, and age 26 and younger. Using induced-exposure methods to gauge accident involvement levels, this analysis showed that drivers over age 75 were overrepresented as the driver at fault in merging and weaving accidents near interchange ramps. With respect to violation types, the older driver groups were cited most frequently for failing to yield and for improper use of lanes. Similarly, Harkey, Huang, and Zegeer's (1996) study of the precrash maneuvers and contributing factors in older driver freeway accidents indicated that older drivers' failure to yield was the most common contributing factor. These data raise concerns about the use of freeway interchanges by older drivers, in light of evidence presented by Lerner and Ratté (1991) that a dramatic growth in older driver freeway travel occurred between 1977 and 1988, with this trend expected to continue.

Age differences in interchange accidents and violations may be understood in terms of driving task demands and age-related diminished driver capabilities. The exit gore area is a transitional area that requires a major change in tracking. A driver (especially in an unfamiliar location) must process a large amount of directional information during a short period of time and at high speeds, while maintaining or modifying his/her position within the traffic stream. When drivers must perform guidance and navigation tasks in close proximity, the chances increase that a driver will become overloaded and commit errors (Lunenfeld, 1993). Erratic maneuvers resulting from driver indecisiveness in such situations include encroaching on the gore area, and even backing up on the ramp or the through lane. When weaving actions are required, the information-processing task demands for freeway interchange maneuvers—both entry and exit—are further magnified.

On a population basis, the age-related diminished capabilities that contribute most to older drivers' difficulties at freeway interchanges include losses in vision and information-processing ability, and decreased physical flexibility in the neck and upper body. Specifically, older adults show declines in static and dynamic acuity, increased sensitivity to glare, poor night vision, and reduced contrast sensitivity (McFarland, Domey, Warren, and Ward, 1960; Weymouth, 1960; Richards, 1972; Pitts, 1982; Sekuler, Kline, and Dismukes, 1982; Owsley, Sekuler, and Siemsen, 1983). These sensory losses are compounded by the following perceptual and cognitive deficits, the first two of which are recognized as being especially critical to safety: reduction in the ability to rapidly localize the most relevant stimuli in a driving scene, reduction in the ability to efficiently switch attention between multiple targets, reduction in working memory capacity, and reduction in processing speed (Avolio, Kroeck, and Panek, 1985; Plude and Hoyer, 1985; Ponds, Brouwer, and van Wolffelaar, 1988; Brouwer, Ickenroth, Ponds, and van Wolffelaar, 1990; Brouwer, Waterink, van Wolffelaar, and Rothengatter, 1991). The most important physical losses are reduced range of motion (head and neck), which impairs visual search, and slowed response time to execute a vehicle control movement, especially when a sequence of movements—such as braking, steering, accelerating to weave and then exit a freeway—is required (Smith and Sethi, 1975; Goggin, Stelmach, and Amrhein, 1989; Goggin and Stelmach, 1990; Hunter-Zaworski, 1990; Staplin, Lococo, and Sim, 1990; Ostrow, Shaffron, and McPherson, 1992).

One result of these age-related diminished capabilities is demonstrated by a driver who waits when merging and entering freeways at on-ramps until he/she is alongside traffic, then relies on mirror views of overtaking vehicles on the mainline to begin searching for an acceptable gap (McKnight and Stewart, 1990). Exclusive use of mirrors to check for gaps and slowing or stopping to look for a gap increase the likelihood of accidents and have a negative effect on traffic flow. Malfetti and Winter (1987), in a critical incident study of merging and yielding problems, reported that older drivers on freeway acceleration lanes merged so slowly that traffic was disrupted, or they stopped completely at the end of the ramp instead of attempting to approach the speed of the traffic flow before entering it. In Lerner and Ratté's (1991) research, older drivers in focus group discussions commented that they experienced difficulty maintaining vehicle headway because of slower reaction times, difficulty reading signs because of visual deficits, fatigue, mobility limitations, a tendency to panic or become disoriented, and loss of daring or confidence. Merging onto the freeway was the most difficult maneuver discussed during the focus group sessions. Needed improvements identified by these older drivers included the elimination of weaving sections and short merge areas, which would facilitate the negotiation of on-ramps at interchanges. Improvements identified to ease the exit process included better graphics, greater use of sign panels listing several upcoming exits, and other methods to improve advance signing for freeway exits.

This section will provide recommendations for highway design elements in four areas to enhance the performance of diminished-capacity drivers at interchanges: A. exit signing and exit ramp gore delineation; B. acceleration/deceleration lane design features; C. fixed lighting installations; and D. traffic control devices for prohibited movements on freeway ramps.

The Handbook recommendations that follow are supported by material presented later in the "Rationale and Supporting Evidence" section under the "Interchanges (Grade Separation)" heading.

Recommendations by Design Element

A.Design Element: Exit Signing and Exit Ramp Gore Delineation

Diagram

* Snowplowable raised pavement markers may be used where appropriate for conditions.

The rationale and supporting evidence for this recommendation can be found by clicking here.

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B.

Design Element: Acceleration/Deceleration Lane Design Features

  1. It is recommended that acceleration lane lengths be determined using the higher of AASHTO (1994) Table X-4 speed change lane criteria or NCHRP 3-35 (Reilly, Pfefer, Michaels, Polus, and Schoen, 1989) values for a given set of operational and geometric conditions, and assuming a 64 km/h (40 mi/h) ramp speed at the beginning of the gap search and acceptance process.
  2. A parallel versus a taper design for entrance ramp geometry is recommended.

    Parallel vs. Taper Design

  3. It is recommended that post-mounted delineators and/or chevrons be applied to delineate the controlling curvature on exit ramp deceleration lanes.

    Post-Mounted Delineators

  4. It is recommended that AASHTO (1994) decision sight distance values be consistently applied in locating ramp exits downstream from sight-restricting vertical or horizontal curvature on the mainline (instead of locating ramps based on stopping sight distance [SSD] or modified SSD formulas).

    The rationale and supporting evidence for this recommendation can be found by clicking here.

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C.Design Element: Fixed Lighting Installations

  1. Complete interchange lighting (CIL) is the preferred practice, but where a CIL system is not feasible to implement, a partial interchange lighting (PIL) system comprised of two high-mast installations per ramp—one fixture on the inner ramp curve near the gore and one fixture on the outer curve of the ramp, midway through the controlling curvature—is recommended.

    The rationale and supporting evidence for this recommendation can be found by clicking here.

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D. Design Element: Traffic Control Devices for Prohibited Movements on Freeway Ramps

  1. The consistent use of 1,200 mm x 750 mm (48 in x 30 in) FREEWAY ENTRANCE signs for positive guidance as described as an option in section 2E-40 of the MUTCD (Federal Highway Administration, 1988), using a minimum letter height of 200 mm (8 in) with series D or wider font, is recommended.
  2. Where adjacent entrance and exit ramps intersect with a crossroad, the use of a median separator is recommended, with the nose of the separator delineated with yellow reflectorized paint and extending as close to the crossroad as practical without obstructing the turning path of vehicles. In addition, it is recommended that a KEEP RIGHT (R4-7a) sign be posted on the median separator nose.
  3. Where DO NOT ENTER (R5-1) and WRONG WAY (R5-9) signs are placed, in accordance with sections 2A-31 and 2E-40 of the MUTCD, a minimum size for R5-1 of 900 mm x 900 mm (36 in x 36 in) and for R5-9 of 1,200 mm x 800 mm (48 in x 32 in) is recommended, with corresponding increases in letter sizes, and the use of high-intensity sheeting. In addition, a mounting height (from the pavement to the bottom of the bottom sign) of between 450 mm and 900 mm (18 in and 36 in) is recommended, using the lowest value for this range that is practical when the presence of snow or other obstructions is taken into consideration.
  4. The application of 7.3-m (24-ft) wrong-way arrow pavement markings (see MUTCD section 2B-20) near the terminus on all exit ramps, accompanied by red raised-pavement markers facing wrong-way traffic, is recommended.

    Wrong Way Arrows

    The rationale and supporting evidence for this recommendation can be found by clicking here.

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