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Road Engineering Journal
Copyright © 1997 by TranSafety, Inc.
November 1, 1997
TranSafety, Inc.
(360) 683-6276
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Reactions of Visually and Physically Impaired Pedestrians to Detectable Warning Surfaces on Sidewalk Curb Ramps

In "Evaluation of Detectable Warning Surfaces for Sidewalk Curb Ramps" (Transportation Research Record 1538), authors Amy A. O'Leary, Philomena B. Lockwood, and Richard V. Taylor discussed the results of a study conducted by the Virginia Department of Transportation (VDOT). The primary purpose of the study was to evaluate the detectability of warning surfaces on sidewalk curb ramps to people with visual impairment and, to a lesser degree, to those with mobility impairment.


The Americans with Disabilities Act (ADA) (July 1990) provided civil rights protection to people with disabilities and covered a number of areas--including public accommodations and transportation. Guidelines were issued for access to buildings and facilities. ADA incorporated a requirement for detectable warning surfaces on sidewalk curb ramps. Although curb ramps are primarily used by people with mobility impairment, pedestrians with visual impairment also use them. For this group, failure to detect a curb ramp poses significant safety concerns. Studies like the one conducted by VDOT may provide data that will help address some safety concerns about detectable warning surfaces.

In March 1992, VDOT adopted a curb-ramp standard that used exposed aggregate as a surface to provide detectable warning. Exposed aggregate is the result of washing away some of the surface cement paste from gravel embedded in concrete. However, for a variety of reasons, VDOT received several requests for exceptions from municipalities that wanted to install curb-ramp surfaces other than exposed aggregate. This, coupled with a nationwide discussion about detectable warning surfaces, prompted VDOT to undertake the study highlighted here.


The study evaluated seven detectable warning surfaces--four with raised truncated domes, one corduroy surface with lateral domes, and two aggregate surfaces. The study involved: (1) a review of literature from 1980 to the present related to detectable warning surfaces, (2) a profile of Virginia residents with visual impairment, (3) field tests of the seven surfaces by participants with visual impairment and a small number with mobility impairment, and (4) a telephone survey of transportation officials in Virginia and other states.

Figures 1 through 7 provide photographs and brief descriptions of the seven surfaces tested in this research project.

Exposed aggregate surface made to Virginia state standard.

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Exposed aggregate surface made with finer gradation gravel than state standard requires.

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Black concrete surface with raised truncated domes (RTDs).

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Concrete surface with lateral domes ("corduroy").

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Red paver bricks with raised truncated domes (RTDs).

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Yellow rubber PathfinderTM tiles with raised truncated domes.

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Yellow composite PathfinderTM tiles with raised truncated domes.

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Photo of test site.

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Photo of Mena Lockwood (author) conducting detection testing with guide dog user.

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Researchers developed a flat test site with uniform sections of the seven detectable surfaces. The surfaces were installed varying distances apart in an existing, rarely used, straight concrete sidewalk at the Virginia Rehabilitation Center for the Blind. Study participants walked the length of the sidewalk twice (down and back) using their usual mobility aids. As soon as they detected a change in the surface, they stopped. Researchers recorded the number of steps participants took on the warning surface and the distance they covered on that surface. When a participant with partial sight saw the warning surface without stepping onto it, the distance from the nearest edge of the surface was recorded. Participants with mobility impairment completed a series of maneuvers on each of the surfaces, and their actions were videotaped. Both groups responded to several subjective questions about each surface.


The literature search revealed recent studies in the U.S. showing "that a variety of surfaces with raised truncated domes are highly detectable to visually impaired people." One U.S. study found domed warning surfaces on ramps presented no serious risk to people with mobility impairments--a significant consideration for state transportation agencies. In the United Kingdom, a concrete corduroy surface was deemed highly detectable.

For the present study, visually impaired participants included 52 percent with partial vision, 10 percent who could only perceive light, and 38 percent with total blindness. State transportation agencies are most concerned about adequate facilities for totally blind pedestrians. Results from the participants with only light perception were not included because of their small number (five). Participants had a variety of mobility skills using guide dogs, canes, and/or sighted guides.

Twenty-six of 27 participants with partial sight detected all the warning surfaces-- except the two exposed-aggregate surfaces. Thirty percent failed to detect the state standard exposed aggregate, and 19 percent failed to detect the smaller gradation aggregate.

For partially sighted participants, results indicated that surfaces with more color contrast were detectable from farther away than the aggregates. The corduroy surface was distinctly visible. The aggregate surfaces provided both less visual and less tactile contrast with the sidewalk than the domed surfaces. In addition, at least 85 percent of the participants rated the five domed surfaces as easy or very easy to detect. In contrast, the two aggregate surfaces were the only ones rated hard or very hard to detect "by appreciable numbers of the partially sighted individuals. . . ."

The 20 totally blind test participants had significant problems detecting the exposed aggregate surfaces--no more than 35 percent detected either one. In contrast, 90 to 95 percent could detect all the domed surfaces except the black concrete dome (80 percent detected this surface). This group of participants nearly always stepped onto the warning surfaces before detecting them. Results suggested that participants detected the rubber domed surface more quickly (it had noticeable resiliency in contrast to the sidewalk) than the black concrete dome surface. Ninety percent or more of this group who rated the black concrete domes, the yellow composite domes, and the red paver bricks felt these surfaces were easy or very easy to detect.

Only six participants with mobility impairment took part in the study (despite extensive recruitment), a sample size too small to yield generalizations. However, "this group consistently preferred the exposed aggregate surfaces, saying that they afforded easier maneuverability than the domed surfaces." These findings were consistent with those from larger studies and indicated that "clearly, there is a fundamental trade-off between surfaces' high detectability for the visually impaired and the ease of maneuverability they afford the mobility-impaired."

Transportation officials from 21 other states responded to the telephone survey. They were asked to report what warning surface, if any, was primarily used in their state and describe the installation/maintenance history of that surface. Several states responded that they were not using any type of detectable warning surface on ramps; several were using brush-finished or broom-finished concrete--which is not a detectable warning surface. Virginia was the only state using exposed aggregate. Some states had experimented with raised truncated domed surfaces, and Washington and North Carolina had required domes in their state standards. Those respondents who had used domed surfaces reported some problems with domes breaking off or domed tiles loosening; freeze-thaw cycles and snowplow damage were cited as the most frequent causes. Virginia respondents reported some installation problems with exposed aggregate, and most felt it was too soon to make predictions on aggregate's long-term performance.


The results of the study "highlight the dilemmas that state department of transportation (DOT) officials face in trying to select optimal warning surfaces for curb ramps." The totally blind test participants failed to detect the exposed aggregate warning surfaces, while those with mobility impairments preferred them. In evaluating detectable warning surfaces for sidewalk curb ramps, overall safety for all users is the primary issue. Current studies may provide more information on detectability and maneuverability. Further studies are warranted on the durability of various types of domed surfaces on ramps used in different areas of the country.

Copyright © 1997 by TranSafety, Inc.

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