Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments - Final Report

3.3 Guidance on the Visual Properties of Detectable Warnings

Based on the results of this study, the following recommendations were developed for the color and contrast of detectable warnings.

• Do not use detectable warnings that are the same color as the sidewalk. The truncated domes by themselves do not provide adequate visual cues for pedestrians with visual impairments. However, low contrast detectable warnings might be sufficient if other methods are be used to increase the visibility of the curb ramp (e.g., if the entire curb ramp contrasts visually with adjacent surfaces), although this study did not investigate such alternatives.

• Select detectable warning color based on the sidewalk color to provide high luminance contrast: either light-on-dark or dark-on-light. A detectable warning that provides a minimum luminance contrast of 60 percent could be seen from a distance of 2.44 m (8 ft) by approximately 92 percent of the pedestrians with visual impairments in our sample under daylight conditions.

• Avoid using combinations of sidewalk and detectable warning materials where the two surfaces providing visual contrast are both dark (reflectance less than 10 percent). For these dark combinations, even relatively high luminance contrast will not ensure high rates of visual detection or conspicuity. On dark sidewalks (e.g. asphalt) use light colored detectable warnings with a high reflectance fact or to provide light-on-dark contrast rather than using darker detectable warnings to provide dark-on-light contrast.

• 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. Two relatively dark surfaces may provide high luminance contrast, but on asphalt or other dark sidewalk surfaces (with reflectance less than 10 percent) high contrast is not always a good predictor of detection and conspicuity.

• 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. Black-and-white or black and federal yellow would likely provide high conspicuity. The pattern elements should be very large relative to the size of the truncated domes. Stripes or other pattern elements should be a minimum of 4inches wide.

• 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. For a given level of luminance contrast, reds and yellows used in this study generally provided higher conspicuity than achromatic colors. Although people who have protan color vision deficiencies (see Appendix A) may not notice the conspicuity enhancement due to the reddish hue, the dark brick red color will retain a dark-on-light appearance and is very likely to be seen against a light-colored curb ramp or other light-colored walking surface. A brick red detectable warning may be better than dark gray or black if it is less likely to be mistaken for a hole or change of elevation.

• Consider how visual contrast between the detectable warning and sidewalk surfaces may change over time as the materials age. For example, concrete generally is lighter when it is new and darkens over time. Asphalt is generally darkest when it is new and lightens over time. Also the detectable warnings materials may fade or darken over time. To the extent possible, these changes should be anticipated so that adequate visual contrast may be maintained as the materials age.