Developing Curb Ramp Designs Based on Curb Radius
By Edward R. Stollof, AICP
AT A RECENT WORKSHOP ON WAYFINDING AT INTERSECTIONS, PARTICIPANTS WERE CHARGED WITH DEVELOPING DESIGN SCHEMES FOR INTERSECTIONS WITH CURB RADII OF 10 FEET, 25 FEET, 30 FEET AND 40 FEET. THE OBJECTIVE WAS TO PLAN FOR AS MUCH REDUNDANCY IN CUEING—TACTILE AND AUDIBLE—AS POSSIBLE.
At a joint workshop of ITE and the U.S. Access Board held in late October 2004, participants were charged with developing design schemes for intersections with curb radii of 10 feet, 25 feet, 30 feet and 40 feet. Each group was asked to consider designs with and without a landscaped strip and with and without signals.
The objective was to plan for as much redundancy in cueing—tactile and audible—as possible. Participants were asked to consider a range of operating conditions (for example, there may or may not be traffic sounds; some of the traffic may be unbalanced).
The primary goal of this exercise was to maximize the information available for non-visual wayfinding at and across the design intersection using curb ramp orientation, edges and other features that might carry directional or travel content.
Participants noted that design for pedestrian use should focus on key users—children, older walkers and pedestrians who have disabilities, particularly those who have vision loss. It was agreed that the standards developed for Sacramento, CA, USA, most nearly met the objectives of the workshop by providing wheelchair-accessible curb ramps that also were in line with the path of pedestrian travel along the sidewalk. Key wayfinding features included short crossings (even if multi-lane); small curb radii; a landscaped parkway; sidewalks and curb ramps in line with crosswalks; and returned edges on curb ramps.
Sponsors and attendees are indebted to workshop participant William F. Hecker Jr., AIA, for the sketches presented in this feature, which were derived from the groups’ work. Ramps generally are described by crossing entry location (apex, return, or tangent); pairing; type (perpendicular, parallel, combination, or in line); and edge condition (flared, curbed, or parkway).
Graphics and Definitions Key
Figure 1 graphically illustrates wayfinding elements at intersections. The representative intersection has a 40-foot radius.
Figure 1. Intersection and curb ramp key; 40-foot radius.
Figure 2 shows the design schemes developed for a 10-foot radius. A small curb radius—found in residential, historic and local business districts—can provide the most directional crossings, with or without a parkway. There is adequate space on the corner for pedestrians to wait and, depending on curb height, usually enough room for paired curb ramps. It is the most economical in terms of space and provides for the least vehicle delay at crossings.
|Figure 2. Design schemes for a 10-foot radius.|
Figure 2a meets the requirements of the current Americans with Disabilities Act (ADA) Accessibility Guidelines for a 2-foot tangent length to provide a directional cue. However, a 17-foot radius is necessary to provide the required 4-foot space in the street at the toe of the ramp to ensure that pedestrians using wheelchairs and scooters do not have to enter a moving lane of traffic to enter the parallel crosswalk.
Pedestrians aligned to the crossing on the tangent also can take a heading from the arrow on the faceplate of the accessible pedestrian signal (APS). However, the crossing is not well aligned with the sidewalk approach direction and pedestrian buttons are not well located for wheelchair users.
Figure 2b ramps the sidewalk down to a level landing at the apex. Crosswalks are an extension of the sidewalk. Pedestrians can take a perpendicular heading from the arrow on the faceplate of the APS. Locating the pedestrian button in the ramp makes it more usable, particularly for pedestrians who are deaf-blind, but requires greater overall sidewalk width. The pedestrian button pole must not impede the clear width needed on the ramp.
Like Figure 2b, Figure 2c ramps the sidewalk down to a level landing at the apex. The returned curbs give good cues to crosswalk direction, which are in line with sidewalk travel. The extra width of the parkway allows more space to be provided for pedestrians on the corner (and would be a good location for APS stub poles, if provided).
In Figure 2d, the full curb height is retained around the apex, with the sidewalk ramping up and down to paired level landings and crosswalks at the tangents.
Like Figure 2d, Figure 2e includes a full-height curb at the apex. The change in elevation is divided into two runs—one in the sidewalk, down to a level landing at the intersection of the two sidewalks, and the second a short perpendicular ramp to the street. In-line travel from sidewalk to ramp to crosswalk and returned edges for wayfinding are key features. Again, the parkway can serve as the location for APS, if provided.
Figure 2f is similar to Figure 2e in that it provides a full curb height corner, but the parkway is wider, permitting greater changes in elevation, if necessary. This alternative is highly directional, with good protection against vehicle overrun at the corner. Perpendicular ramps located on the curb return must have a grade break at the toe rather than the curbline.
Participant Comments on Figure 2
Figure 3 shows the design schemes developed for a 25-foot radius. A 25-foot radius usually is found in a local residential, local commercial, or central business district situation. There will be a 5- to 6-foot sidewalk behind the curb or separated by a parkway.
Figure 3. Design schemes for a 25-foot radius.
Figure 3a ramps the sidewalk down at each tangent point, with the entire curb radius depressed to street level. Sidewalk travel is almost in line, but there are few other cues to crossing direction. APS must be carefully aligned and well separated to give any wayfinding input.
Figure 3b shows that the level landings are set at each tangent and the sidewalk is then ramped up and down, something of a disadvantage for pedestrians unless curb height is low. However, it does preserve a full-height curb at the corner. An APS locator tone could help identify the crossing locations near the tangent part of the curve.
Participant Comments on Figure 3a
Participant Comments on Figure 3b
Figure 4 shows the design schemes developed for a 30-foot radius, perhaps along a major street or arterial. There will be a 5- to 6-foot sidewalk behind the curb, rarely separated by a parkway.
The working group’s first objective, in new construction or reconstruction, was to reduce this radius, if feasible. The group believed that the majority of the time, the radius might not need to be 30 feet. Vehicles should use the effective turning radius. If there is on-street parking, vehicles do not need to turn from curb lane to curb lane. The engineer must measure the radius and select a design vehicle. Vehicles must be able to turn, but they can be permitted to go across the centerline of the local street.
Figure 4. Design schemes for a 30-foot radius.
Figure 4a features a narrow parkway and preserves the full curb height around the corner. Paired parallel curb ramps (they could be combination ramps with a slope at the detectable warnings) are fitted with landings at the tangent and have returned edges for directionality. The APS is at the back of the sidewalk, but could be at the back edge of the parkway for superior wayfinding. The enclosed space of the level landing must accommodate a 90-degree turn; it is very tight.
Figure 4b, with a wider border and parkway, permits paired perpendicular ramps at the tangents. Good directionality is provided for pedestrians who use the ramps, but the ramps are well out of the path of sidewalk travel. A full-height curb can be provided on the radius; APS are well located for directionality in the parkway. APS stub poles also could be used in lieu of the flare to shield a returned curb on the other side of the ramp.
Participant Comments on Figures 4a and 4b
Figure 4c collapses the intersection to more recognizable dimensions. The sidewalk is ramped down to a level landing at the apex and a shorter ramp (with detectable warnings) connects to the street (this also could be level rather than ramped if all of the elevation change occurs in the parallel ramps of the sidewalk).
A small length of returned curb can suggest directionality. A somewhat longer opening at the apex might be able to accommodate a raised triangle to separate the ramp into two. The APS at the parkway is well located for all users and close enough to the street to provide usable directional cues as well as the 10-foot separation needed to distinguish signals.
Figure 4d locates the ramp in line with the sidewalk for superior wayfinding benefit and overcomes the bad geometry of a non-perpendicular connection at the toe by notching the curb line. A wide parkway provides ample space for well-located APS at the top of the ramp. The apex is raised to full curb height. Perpendicular ramps on the curb return must have a grade break at the toe rather than the curbline.
Participant Comments on Figure 4c
Participant Comment on Figure 4d
Figure 4e provides for a single crossing beside the major roadway of a minor road without sidewalks—a common type that poses the greatest wayfinding design challenge. The large radius places waiting pedestrians where they may not be seen by drivers or understood to be waiting to cross. Sound cues from vehicles on the far side of the minor street may not be audible.
Ideally, a detectable warning would parallel the curb line, but that may be difficult in installations like these. However, the detectable warning should not be installed too far from the curb. The in-line crossing is a benefit. Returned curbs give good directional cues. Perpendicular ramps on the curb return must have a grade break at the toe rather than the curbline.
An alternative to Figure 4e is Figure 4f (for large curb radii), which places the detectable warning in a triangular “landing” at the toe of the ramp. The inline ramp ends in a perpendicular grade break at the toe, with a substantial notch in the curb line. Returned curbs give good direction and may offer some protection to waiting pedestrians, who are closer to the street edge and in a much better position to analyze traffic sounds for crossing information. Perpendicular ramps on the curb return must have a grade break at the toe rather than the curbline.
Participant Comments on Figures 4e and 4f
Figure 1 shows a 40-foot radius. This design proposes a range of ramp types at the tangents. Like the 30-foot designs, this places crossings well out of the line of continuous travel. In fact, these are much like the crossing conditions at roundabouts. Benefits for pedestrians are the much shorter—and, therefore, quicker—crossings with less pedestrian exposure and more cueing for directionality.
(These benefit drivers, too, by lessening delay.)
Apex crossings at these radii might be so long as to make it difficult to avoid veering. Ramp choices are varied, from the standard perpendicular, where there is a generous border for a parkway of 6 feet or so, to combination ramps with some elevation change accomplished by ramping the sidewalk down and the remaining difference treated with a short perpendicular run to the street. Again, bar tiles are suggested to identify crossing locations (these also have been suggested at roundabouts). APS are located in the parkway at the landing or at the top of the perpendicular ramp.
|Figure 5. Raised crossing (speed table).|
Where feasible, raised crossings can eliminate the need for curb ramps and are a good choice for narrow sidewalks (see Figure 5). The ramped sides of the table may be useful cues to the crosswalk. Often, a bridge plate spans the gutter to allow drainage patterns to continue, as detectable warnings are necessary at these flush connections.
Bulbouts also can be used where border width is inadequate for a ramp. Curb radii can be limited and used in concert with the width of the parking lane to provide shorter crossings that are still manageable by long-wheelbase vehicles when turning.
These comments and sketches represent a first cut at the workshop goal of developing standard sidewalk and street crossing details that maximize non-visual directional and orientation information.
The U.S. Access Board will be pursuing these discussions online within the group of workshop attendees and circulating the range of design options described in this feature for further consideration and improvement. As consensus is reached, recommendations will be documented and presented to ITE and other industry audiences at scheduled meetings and conventions.
For more information, to submit other drawings or recommendations, or to be added to the list of participants, contact Lois Thibault, research coordinator, U.S. Access Board, at email@example.com.
The author would like to thank William F. Hecker Jr., of Hecker Design Ltd., for developing the illustrations incorporated into this feature, and Janet M. Barlow, certified orientation and mobility specialist, of Accessible Design for the Blind, for her review of each concept plan.
EDWARD R. STOLLOF, AICP, is contracts senior director and manager of the safety discipline at ITE Headquarters. He administers ITE’s external contracts and grants and is the designated staff person involved in ITE’s safety mega-issue activities. He has more than 25 years of broad transportation planning and traffic engineering experience in both the public and the private sectors. He is a member of the Howard County, MD, USA, Public Transportation Board and is a member of ITE.