Executive Summary

Detectable warnings are walking surfaces that are primarily intended to provide a tactile cue to pedestrians who are visually impaired. They are installed at locations such as the edge of a train platform or at the transition between the sidewalk and the street to warn pedestrians of the potential hazard that lies ahead. The tactile properties of detectable warnings result from a grid of small, truncated (flat-topped) domes across the warning surface. This pattern has been standardized by the U.S. Access Board and testing has shown that the pattern can be detected underfoot or by cane without causing a tripping hazard or obstructing wheelchairs. Despite the proven tactile benefits of detectable warnings, little research has been conducted to evaluate the visual detectability of various detectable warning materials. Detectable warnings that provide salient visual cues in addition to tactile cues may help many pedestrians with visual impairments to locate hazards or curb ramps from a greater distance than is possible using the tactile cues alone. Some pedestrians may use them to orient to a curb cut or ramp at the end of a crosswalk.

The objectives of this study were (1) to determine which detectable warning colors and patterns are visually detectable and conspicuous to pedestrians with visual impairments and (2) to provide recommendations related to color, pattern, and luminance contrast of detectable warnings for placement on sidewalks.

Fifty men and women ranging in age from 24 to 92 participated in this study. All participants had impaired but useful vision. Most were legally blind. All participants reported they had difficulty locating the boundary between sidewalks and streets.

Thirteen detectable warnings were tested. The set included ten uniform colors (white, simulated white concrete, simulated brown concrete, light gray, dark gray, bright federal yellow, pale yellow, bright red, orange-red, and black) and three black-and-white patterns. Each detectable warning was a .91 m (3 ft) wide by .61 m (2 ft) long composite panel designed for surface application. Participants viewed each detectable warning on four different horizontal backgrounds. Each background was 1.22 m (4 ft) wide by 2.44 m (8 ft) long and was constructed to simulate the appearance of a red brick sidewalk, a dark gray asphalt sidewalk, a white concrete sidewalk, and a brown concrete sidewalk. The study was conducted during midday hours with dry surfaces.

Participants viewed each combination of detectable warning and sidewalk individually. To determine detection distance, participants first viewed the sidewalk from 7.92 m (26 ft) away and, if they could not see the detectable warning from this distance, they began to walk closer until they were confident that a detectable warning was present. On some trials there was no detectable warning present. Once detectable distance had been measured, participants were asked to view the detectable warning from a distance of 2.44 m (8 ft) and to describe the color and/or pattern of the detectable warning. Finally, participants were asked to rate the conspicuity (attention-getting property) of the detectable warning on a five-point scale.

Participants viewed each combination of detectable warning and background color individually. To determine detection distance, participants first viewed the simulated sidewalk section from 7.92 m (26 ft) away and if they could not see the detectable warning from this distance, they began to walk closer until they were confident that a detectable warning was present. On some trials there was no detectable warning present. After detection distance had been measured, participants viewed the detectable warning from a distance of 2.44 m (8 ft) and described its color, and rated the conspicuity (attention-getting property) of the detectable warning on a five-point scale.

Detection distance results indicate that pedestrians with visual impairments were able to see most combinations of detectable warning and sidewalk from 2.44 m (8 ft) away, but were less likely to see them from 7.92 m (26 ft) away. Detectable warnings that were similar in color to the sidewalk were seen by few participants, indicating that visual cues provided by the truncated dome pattern itself are not sufficient to ensure visual detection. The color of the sidewalk strongly influenced how easily single-color detectable warnings could be seen; however, black-and-white patterned detectable warnings were visually detectable and conspicuous for most participants across all sidewalk types. The luminance contrast provided by the detectable warning and the sidewalk (or by the patterns) was an important factor for predicting the likelihood that a detectable warning would be seen. Where luminance contrast was 70 percent or greater, about 95 percent of participants were able to see the detectable warning from 2.44 m (8 ft) away. Detectable warnings that provided at least 60 percent contrast could be seen by about 92 percent of participants from 2.44 m (8 ft) away. Dark detectable warnings on a dark sidewalk were an exception. Although providing moderately high luminance contrast, these combinations were detected less often than would be predicted from their luminance contrast.

Detection distance results indicate that pedestrians with visual impairments were able to see most combinations of detectable warning and sidewalk from 2.44 m (8 ft) away, but were less likely to see them from 7.92 m (26 ft) away. Detectable warnings that were similar in color to the sidewalk were seen by few participants, indicating that visual cues provided by the truncated dome pattern itself are not sufficient to ensure visual detection. The color of the sidewalk strongly influenced how easily single-color detectable warnings could be seen, however, black-and-white patterned detectable warnings were visually detectable and conspicuous for most participants on all sidewalk colors tested. The luminance contrast provided by the detectable warning and the sidewalk (or by the patterns) was an important factor for predicting the likelihood that a detectable warning would be seen. Where luminance contrast was 70 percent or greater, about 95 percent of participants were able to see the detectable warning from 2.44 m (8 ft) away. Detectable warnings that provided at least 60 percent contrast could be seen by about 92 percent of participants from 2.44 m (8 ft) away. Dark detectable warnings on a dark sidewalk were an exception. Although providing moderately high luminance contrast, these combinations were detected less often than would be predicted from their luminance contrast. For dark single-color detectable warnings and black-and-white patterned detectable warnings a few participants commented that the detectable warning looked like something else (e.g. hole, metal grate).

Besides luminance contrast, regression analyses indicated that some other characteristics of detectable warnings were generally associated with high detection rates and high conspicuity ratings. These include color (reds and yellows rather than achromatic) and reflectance (lighter colors rather than darker colors). For the range of conditions tested, neither illumination level (per trial) nor sky conditions (percent cloud cover per session) affected detection and conspicuity of detectable warnings.

Based on the results of the study, the authors recommend the following:

  • Do not use detectable warnings that are the same color as the sidewalk.
  • Select detectable warning color based on the sidewalk color to provide high luminance contrast either light-on-dark or dark-on-light.
  • Avoid using combinations of sidewalk and detectable warning materials where both surfaces are dark (reflectance less than 10 percent).
  • If a contrast-based requirement for detectable warnings installations is used, the guidance should include both a minimum luminance contrast and a minimum reflectance for the lighter of the two surfaces providing the contrast.
  • If a standardized color scheme is desired for detectable warnings, adopt a two-color large pattern which provides high internal contrast to ensure high conspicuity across all sidewalk types.
  • If a standardized color scheme is desired for single-color detectable warnings, federal yellow may be a good choice. It provides a high level of conspicuity for a given level of luminance contrast. In this study reds and yellows generally provided higher conspicuity than achromatic colors.
  • If a small set of standardized colors is desired for detectable warnings on different sidewalk types then federal yellow may be a good choice where adjacent walking surfaces are dark. A dark brick red color (orange-red) may be a good choice where adjacent walking surfaces are light.
  • Consider how visual contrast between the detectable warning and sidewalk surfaces may change over time as the materials age.

Further visibility testing of detectable warnings should include a broader range of lighting conditions (dusk, dawn, artificial illumination), determination of optimal internal contrast patterns for two-color detectable warnings, and viewing detectable warnings in naturalistic roadway environments with unpredictable crossing locations, distractions, visual obstructions, wet surfaces, and so forth. Further research also should include pedestrians’ perceptions of different detectable warning colors (e.g. Is the detectable warning recognized as being safe to step on? Does the detectable warning convey the intended message?).