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Publication Number: FHWA-RD-01-103
Date: May 2001

Highway Design Handbook for Older Drivers and Pedestrians

II. INTERCHANGES (GRADE SEPARATION)

 

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 crashes there is four times greater than at any other freeway location. Two decades later, Lunenfeld (1993) reiterated that most freeway crashes and directional uncertainty occur in the vicinity of interchanges.

Distinct patterns in the occurrence of freeway interchange crashes emerge in studies that look specifically at driver age. Staplin and Lyles (1991) conducted a statewide (Michigan) analysis of the crash involvement ratios and types of violations for drivers in four age groups: age 76 and older; ages 56 to 75; ages 27 to 55; and age 26 and younger. Using induced-exposure methods to gauge crash involvement levels, this analysis showed that drivers over age 75 were overrepresented as the driver at fault in merging and weaving crashes 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 study (1996) of the precrash maneuvers and contributing factors in older driver freeway crashes indicated that older drivers were much more likely than younger drivers to be merging or changing lanes, or passing/overtaking prior to a crash, and 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. Broader demographic and societal changes suggest that the dramatic growth in older driver freeway travel between 1977 and 1988 reported by Lerner and Ratté (1991) will continue and even accelerate in the years ahead.

Age differences in interchange crashes 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 they 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 both entry and exit maneuvers 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, and 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 crashes 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 the mainline. In a survey of 692 older drivers, 25 percent reported that they stop on a freeway entrance ramp before merging onto the highway, and 17 percent indicated that they have trouble finding a large enough gap in which to merge onto the mainline (Knoblauch, Nitzburg, and Seifert, 1997). Thirty-four percent of the "young-old" respondents (ages 50 to 72) and 26 percent of the "old-old" respondents (ages 73 to 97) responded that they wish entrance lanes were longer. In Lerner and Ratté's research (1991), older drivers in focus group discussions commented that they experienced difficulty maintaining vehicle headway because of slower reaction times, difficulty reading signs, 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. 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 restricted or prohibited movements on freeways, expressways, and ramps.

 

Recommendations by Design Element

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

ITE:4
MUTCD:4

(1) The calculation of letter size requirements for signing at interchanges and on their approaches based on an assumption of not more than 10 m (33 ft) of legibility distance for each 25 mm (1 in) of letter height is recommended for new or reconstructed installations and at the time of sign replacement.
MUTCD:2

(2) To increase the reading distance of all highway destination signs, it is recommended that a mixed-case font, as presently used for overhead installations, also be used for ground-mounted signs on the side of the road (e.g., MUTCD sign numbers D1-1 to D1-3).

IEC: requires
FHWA permission

(3) A modification of upstream diagrammatic guide signing as displayed in the MUTCD (figure 2E-7) is recommended for new or reconstructed installations, whereby the number of arrow shafts appearing on the sign matches the number of lanes on the roadway at the sign's location (as shown below).

Existing MUTCD format
Existing MUTCD format
Recommended alternative
Recommended alternative
AASHTO:4
MUTCD:4
(4) It is recommended that:

(4a) Delineation in the vicinity of the exit gore at nonilluminated and partially illuminated interchanges include, as a minimum, the treatments illustrated in the figure below:

Note: Figure is not to scale.

Diagram of exit interchange

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

(4b) Where engineering judgment has identified a hazardous gore area (e.g., containing a ditch) or other special visibility need, the minimum treatment indicated in Recommendation IIA (4a) and illustrated in the figure above should be supplemented by a Type 3 object marker (OM-3C).

The rationale and supporting evidence for these recommendations

 

  B. Design Element: Acceleration/Deceleration Lane Design Features
AASHTO:4 (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 values for a given set of operational and geometric conditions, and assuming a 65-km/h (40-mi/h) ramp speed at the beginning of the gap search and acceptance process.
AASHTO:4 (2) A parallel versus a taper design for entrance ramp geometry is recommended.
MUTCD:3 (3) It is recommended that post-mounted delineators and/or chevrons be applied to delineate the controlling curvature on exit ramp deceleration lanes.
AASHTO:2

(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 these recommendations

 

  C. Design Element: Fixed Lighting Installations
AASHTO:4
RLH:4

(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 (e.g., 18- to 46-m- [60- to 150-ft-] high structures with 3 to 12 luminaires per structure) per ramp is recommended, with one fixture located on the inner ramp curve near the gore, and one fixture located on the outer curve of the ramp, midway through the controlling curvature.

The rationale and supporting evidence for this recommendation

 

  D. Design Element: Traffic Control Devices for Restricted or Prohibited Movements on Freeways, Expressways, and Ramps
MUTCD:4

(1) To increase the legibility distance of overhead lane control signal indications for prohibited movements (red X), a double-stroke arrangement of pixels that are small (approximating a 4 mm diameter) and closely spaced (approximating 18 mm, center-to-center) is recommended.

MUTCD:4 Freeway Entrance sign (2) The consistent use of a 1200-mm x 750- mm (48-in x 30-in) guide sign panel with the legend FREEWAY ENTRANCE, using a minimum letter height of 200 mm (8 in) for positive guidance, as described as an option in section 2E.50 of the MUTCD (FHWA, 2000) and shown to the right, is recommended.
MUTCD:4 (3)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 retroreflectorized markings and extending as close to the crossroad as practical without obstructing the turning path of vehicles. Where engineering judgment determines the need for the median nose to be set back from the intersection, the setback distance should be treated by a 300-mm (12-in) or wider yellow stripe bordered by yellow ceramic buttons that are touching throughout the length of the setback. In addition, it is recommended that a KEEP RIGHT (R4-7a) sign be posted on the median separator nose.
  (4) To meet overriding concerns for enhanced conspicuity of signing for prohibited movements, the following countermeasures are recommended where DO NOT ENTER (R5-1) and WRONG WAY (R5-1a) signs are used:
MUTCD:4

(4a) A minimum size for R5-1 of 900 mm x 900 mm (36 in x 36 in) and 1200 mm x 800 mm (48 in x 32 in) for R5-1a is recommended, with corresponding increases in letter size.

MUTCD:1

(4b) To provide increased sign conspicuity and legibility for older drivers, retroreflective fluorescent red sheeting materials that provide for high retroreflectance overall, and particularly at the widest available observation angles, are recommended.

MUTCD:4

(4c) Where engineering judgment indicates an exaggerated risk of wrong-way movement crashes, it is recommended that both the R5-1 and R5-1a signs be installed on both sides of the ramp, placed in accordance with the MUTCD.

IEC: requires FHWA permission

(4d) Where all other engineering options have been tried or considered, lowering sign height* to maximize brightness under low-beam headlight illumination is recommended by mounting the signs 900 mm (36 in) above the pavement (measured from the road surface to the bottom of the sign), or the lowest value above 900 mm that is practical when the presence of snow, vegetation, or other obstructions is taken into consideration.

* This does not meet the standards set by the by MUTCD Section 2A.18 or the Roadside Design Guide Section 4.3.3, so the reasons for choosing to implement this recommendation should be clearly documented by the authorized agency.

MUTCD:3

(5) The application of 7.1-m- (23.5-ft-) long wrong-way arrow pavement markings (see MUTCD section 3B.19, figure 3B-22) near the terminus on all exit ramps is recommended.

The rationale and supporting evidence for these recommendations

 

FHWA-RD-01-103

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