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Handbook for Designing Roadways for the Aging Population

Chapter 3. Interchanges

This section of the Handbook provides treatments for highway design elements in six areas to enhance the performance of aging drivers at interchanges. Also, after the last design element, two promising practice treatments are presented. Drawings are for illustrative purposes only; they are not to scale and should not be used for design purposes.

Proven Practices

  1. Exit Signs and Markings
  2. Freeway Entrance Traffic Control Devices
  3. Delineation
  4. Acceleration/Deceleration Lane Design
  5. Interchange Lighting
  6. Restricted or Prohibited Movements

Promising Practices

  1. Route Shield Markings at Major Freeway Junctions
  2. Wrong-Way Driving Countermeasures

Overall, freeways are characterized by the highest safety level (lowest fatality rates) when compared with other types of highways in rural and urban areas (AAA Foundation for Traffic Safety, 1995). At the same time, freeway interchanges have design features that have been shown to result in safety and operational issues. Taylor and McGee (1973) reported 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 pre-crash maneuvers and contributing factors in aging driver freeway crashes indicated that aging drivers were much more likely than younger drivers to be merging or changing lanes, or passing/overtaking prior to a crash, and that aging drivers' failure to yield was the most common contributing factor. These data raise concerns about the use of freeway interchanges by aging drivers. Broader demographic and societal changes suggest that the dramatic growth in aging 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 aging drivers' difficulties at freeway interchanges include losses in vision and information-processing ability, and decreased physical flexibility in the neck and upper body. Specifically, aging adults show declines in static and dynamic acuity, increased sensitivity to glare, poor night vision, and reduced contrast sensitivity (McFarland, et al., 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, et al., 1990; Brouwer, et al., 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 aging 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 aging 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 respondents ages 50 to 72, and 26 percent of the respondents ages 73 to 97, responded that they wish entrance lanes were longer. In Lerner and Ratté's research (1991), aging 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 aging drivers included the elimination of weaving sections and short merge areas, which would facilitate the negotiation of onramps 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.

The intersection of interchange entrance and exit ramps with surface streets often creates an environment that is unfriendly and sometimes unsafe for pedestrians. Motorists often have a low yield rate at these locations and are traveling at moderately high speeds. These factors place vulnerable pedestrians, including aging pedestrians, in a compromised state when crossing at these locations. There has been very little research performed on specific roadway treatments for pedestrians at interchanges. As a result, there are not specific treatments included in this chapter. However, the treatments included in Chapter 2 for intersections may be considered and applied where engineering judgment deems it appropriate to increase driver awareness of pedestrians, slow vehicle speeds, and improve pedestrian mobility.

Proven Practices
25. Exit Signs and Markings

A. Letter Size

The calculation of letter size requirements for signs at interchanges and on their approaches based on an assumption of a minimum specific ratio of 1 inch of letter height per 30 feet of legibility distance is recommended for new or reconstructed installations and for sign replacement.

References *: TEH:4, MUTCD:1

B. Mixed-Case Lettering

To increase the reading distance of all highway destination signs, it is required by the 2009 MUTCD that mixed-case lettering be used for destination and street names.

References *: MUTCD:1

C. Overhead Arrow-per-Lane Sign

The MUTCD recommends Overhead Arrow-per-Lane guide signs to be used on all new or reconstructed freeways and expressways as described in MUTCD Sections 2E.20 and 2E.21, whereby the number of arrow shafts appearing on the sign matches the number of lanes on the roadway at the location of the sign (see Figure 42).

Figure 42. An image of an overhead freeway exit sign for a three-lane approach to Exit 11.  The left lane continues straight toward I-595 East and Annapolis, the right lane is an exit-only lane to Route 197 and Mitchellville, and the middle lane has the option of either lane.  An arrow above each lane on the sign indicates those directions of travel for approaching drivers.

Figure 42. Example Overhead Arrow-per-Lane Sign

Freeway and expressway splits or multi-lane exit interchanges that contain an interior option lane in which traffic can either leave the route or remain on the route, or choose either destination at a split, from the same lane should use overhead arrow-per-lane guide signs rather than diagrammatic guide sign designs. Overhead arrow-per-lane guide signs have been shown to be superior to either conventional guide signs or diagrammatic guide signs because they convey positive direction about which destination and direction each approach lane serves, particularly for the option lane, which is otherwise difficult to clearly sign.

References *: MUTCD:1

D. Retroreflective Sheeting

Microprismatic retroreflective sheeting should be used on overhead and ground-mounted guide signs (in place of Type III sheeting). The use of Clearview® font, in conjunction with retroreflective sheeting, should be considered because it may further enhance positive contrast legends if used appropriately.

References *: MUTCD:4, MUTCD:5

The rationale and supporting evidence for these treatments can be found beginning on page 224 of this Handbook.

26. Freeway Entrance Traffic Control Devices

A. Guide Sign

A 48-in x 30-in guide sign panel with the legend Freeway Entrance (see Figure 43), using a minimum letter height of 8 in, should be consistently used in situations where freeway entrance and exit ramps are adjacent to one another (such as at a partial cloverleaf interchange) and placed as described in Section 2D.46 and shown in Figure 2D-14 of the MUTCD.

References *: MUTCD:1

Figure 43. An image of a 'FREEWAY ENTRANCE' (MUTCD D13-3) sign.

Figure 43. (MUTCD D13-3)

B. Adjacent Entrance/Exit Ramps

Where adjacent entrance and exit ramps intersect with a crossroad, the use of a median separator, either painted or preferably raised, 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 (see Figure 44). 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 12-in or wider yellow stripe. In addition, a KEEP RIGHT (R4-7) sign should be posted on the median separator nose, if it is raised.

References *: MUTCD:4

Figure 44. An image of an intersection between a local street and a freeway access point where the exit and entrance are adjacent.  The image shows recommended placement of signs and pavement markings.

Figure 44. Recommended signs and markings for adjacent entrance/exit ramps at a crossroad intersection

C. Diagrammatic Entrance Sign

For diagrammatic guide signs depicting lane use for entry to a freeway from an urban multilane arterial, maximum visibility is achieved through overhead sign placement. Where this is not feasible, two advance ground-mounted diagrammatic guide signs should be used, one placed at 0.5 mi and the second placed at 0.25 mi in advance of the interchange (see Figure 45).

References *: MUTCD:4

Figure 45. An image of a sign showing a diagram of two upcoming turns.  The nearest is a left turn toward west I-270 and Dublin; subsequently there is a right turn toward east I-270 and Cleveland.  Below the diagram is the notation '1/4 MILE'.

Figure 45. Advance ground-mounted diagrammatic sign

The rationale and supporting evidence for these treatments can be found beginning on page 235 of this Handbook.

27. Delineation

A. Delineators/Raised Pavement Markers

Delineation in the vicinity of the exit gore at non-illuminated and partially illuminated interchanges should include, as a minimum, raised pavement markers and retroreflective post-mounted delineators as shown in Figure 46.

References *: Green Book:4, MUTCD:4

Figure 46. An image of the approach to a freeway exit ramp showing the recommended location of post-mounted delineators along the exit ramp and in the gore, as well as an object marker at the beginning of the gore and raised pavement markers on the painted markings defining the painted gore area.

Figure 46. Recommended raised pavement markers and post-mounted delineators at an exit gore

B. Object Marker

Where engineering judgment has identified a hazardous gore area (e.g., containing a ditch) or other special visibility need, the minimum treatments described above should be supplemented by adding Type 1 object markers to the exit gore sign post as illustrated in Figure 46.

References *: Green Book:4, TEH:3, MUTCD:1

C. Chevrons/Post-Mounted Delineators

Post-mounted delineators and/or chevrons should be applied to delineate the controlling curvature on exit ramps, as illustrated in Figure 47.

References *: MUTCD:3

Figure 47. An image of a freeway exit ramp showing the placement of post-mounted chevrons along the left side of exit ramp around the sharpest curve.

Figure 47. Placement of chevrons on the controlling curve of an exit ramp

The rationale and supporting evidence for these treatments can be found beginning on page 238 of this Handbook.

28. Acceleration/Deceleration Lane Design

A. Entrance Ramp Geometry

A parallel (rather than a taper) design for entrance ramp geometry is recommended, as shown in Figure 48. AASHTO recognizes the operational and safety benefits of long acceleration lanes provided by parallel type entrances. A long acceleration lane provides more time for merging drivers to find an opening in the through-traffic stream. A parallel style entrance lane length of at least 1,200 ft, plus a taper, is desirable.

References *: Green Book:1

Figure 48. An image of two options for the design and pavement markings for an acceleration lane for a freeway entrance ramp.  The option on the left shows a parallel acceleration lane, and the option on the right shows a tapered acceleration lane.  The option on the left is noted as 'PREFERRED'.

Figure 48. Recommended markings for acceleration lanes from entrance ramps onto freeways

B. Location of Exit Ramps

The AASHTO (2011) decision sight distance values should 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 formulas).

References *: Green Book:2

The rationale and supporting evidence for these treatments can be found beginning on page 243 of this Handbook.

29. Interchange Lighting

A. Complete versus Partial Lighting

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., 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.

References *: Green Book:4, RLH:4

The rationale and supporting evidence for this treatment can be found beginning on page 248 of this Handbook.

30. Restricted or Prohibited Movements

A. Signing Practices

To meet overriding concerns for enhanced conspicuity of signing for prohibited movements, the following countermeasures should be used where DO NOT ENTER (R5-1) and WRONG WAY (R5-1a) signs are used:

B. Pavement Markings

References *: MUTCD:3

The rationale and supporting evidence for these treatments can be found beginning on page 252 of this Handbook.

Promising Practices

These are treatments being utilized by transportation agencies that should benefit aging road users as determined by a subjective assessment by staff participating on the development of this Handbook. Current trends indicate these practices have a positive impact on aging road user safety.

31. Advance Pavement (Route Shield) Markings at Major Freeway Junctions

At major freeway interchanges and route splits, route shield markings should be used in the lanes approaching the split to guide drivers to the correct approach lane (see Figure 50). The placement of this type of marking should be just prior to the location of the advance guide signs.

References *: MUTCD:1

Figure 50. An picture of a section of freeway with route shield markings painted within the lanes.  The left two lanes have an I-30 shield, and the right two lanes have an I-35E shield.

Figure 50. Route Shield Markings At Freeway Junctions

The rationale and supporting evidence for these treatments can be found beginning on page 257 of this Handbook.

32. Wrong-Way Driving Countermeasures

The NTSB's FARS analysis determined that drivers over the age of 70 are over-represented in fatal wrong-way crashes (NTSB, 2013). Additional treatments to counter wrong-way driving by aging drivers (e.g., improved lighting, channelization, signs and markings in addition to those in Treatment 30) should be considered where exit ramps intersect with surface streets. Road owners could employ the use of a Road Safety Audit (RSA) to examine the performance of the interchange and determine appropriate countermeasures to employ. A Wrong-Way Driving (WWD) Prompt list is available to focus specific attention on wrong-way driving issues and contributing factors. The prompt list has been developed in a similar framework to the broader RSA prompt lists contained in Chapter 8 of the FHWA RSA Guidelines document. The prompts are only an aid to the RSA team and they are not intended to cover all conditions or circumstances an RSA team may encounter. The American Traffic Safety Services Association also provides a publication that describes promising practices in wrong-way driving countermeasures (ATSSA, 2012); a review of that document should be included during the consideration of potential WWD treatments to implement.

References *: MUTCD:4

The rationale and supporting evidence for these treatments can be found beginning on page 257 of this Handbook.

References Legend

See pages 3 and 4 for full description of codes and acronyms of cited design guides.

Page last modified on October 15, 2014.
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