3.1 Key Findings

  • There were many combinations of detectable warning color and sidewalk color that were seen from a distance of 2.44 m (8 ft) by pedestrians with visual impairments, but there were fewer combinations seen from 7.92 m (26 ft) and fewer that were rated highly in their ability to attract pedestrians’ attention. Forty-one of the 52 combinations tested were seen by more than 85 percent of the participants from 2.44 m (8 ft), 14 of 52 combinations were seen by more than 85 percent of participants from 7.92 m (26 ft), and only 3 of the combinations received high conspicuity ratings from more than 85 percent of the participants.
  • Detectable warnings that are the same color as the sidewalk or very similar in color to the sidewalk could not be seen by most participants in this study.
  • For most detectable warning colors tested, the color of the sidewalk upon which the detectable warning was placed influenced how easily it could be seen. An exception to this was the high contrast black-and-white patterned detectable warnings which were generally detectable and conspicuous across all four sidewalk types.
  • The luminance contrast between the detectable warning and sidewalk (particularly the logarithm of contrast) was an important factor for predicting the percentage of participants with visual impairments who were able to see the detectable warning. At contrasts above 70 percent, detectable warnings were seen from 2.44 m (8 ft) by approximately 95 percent of the participants. At contrasts above 50 percent, more than 90 percent of participants were able to see the detectable warning at 2.44 m (8 ft). The only exception to this was the black detectable warning on the asphalt sidewalk which had a dark-on-light contrast of 68 percent. It was seen by fewer than 80 percent of the participants.
  • Regression analyses show that in addition to luminance contrast, other factors may be important predictors of visual detection and conspicuity for detectable warnings. In particular, there were differences between chromatic and achromatic detectable warnings. The four red and yellow detectable warnings (bright red, orange-red, federal yellow, and pale yellow) generally provided greater conspicuity and greater probability of detection than achromatic detectable warnings for a given level of luminance contrast. For predicting high conspicuity ratings the reflectance of the detectable warning is helpful. Based on parameter estimates for the regression models, having lighter detectable warnings (higher reflectance) predicts high conspicuity ratings.
  • Regression analyses show no evidence that the range of lighting conditions (cloud cover, illuminance) tested in this study influence detection or conspicuity of detectable warnings.
  • Participants’ descriptions of detectable warning colors sometimes changed with sidewalk type, although most color descriptions given were consistent with the perceptions of the experimenters (who had no visual impairment). Some participants’ use of color names was clearly inconsistent with other participants’ descriptions, indicating variability in color perception for detectable warnings.
  • Participants’ unsolicited comments about the suitability of various detectable warnings were recorded and are included in Appendix G. Some of the comments focused on the problem that certain detectable warnings may look like other things commonly encountered on sidewalks such as holes, patches, or debris.

3.2 Study Limitations and Other Issues
The present study was limited somewhat by the testing environment. Three out of four of the simulated sidewalk sections used as backgrounds for the detectable warnings were not actual paving materials, but were simulated from paint and sand mixtures, and from asphalt roofing material. The optical properties of actual paving materials (concrete, asphalt) may provide different visual cues than those provided by the simulated sidewalk surfaces. Reflectance and chromaticity of real sidewalks vary widely in their reflectance and chromaticity at different locations, and it was possible to represent only a limited range of this variation across the four simulated sidewalks produced for this study. We have provided reflectance and chromaticity measurements for all surfaces used in this study to aid in comparing the present results to those from other studies.

The procedures used in this study were designed to test only visual detection, conspicuity and color appearance of detectable warnings. Therefore, participants were never asked to step on any of the detectable warnings. It is possible that certain combinations of detectable warnings and sidewalk colors, although visually conspicuous may be mistaken for some walking hazard such as a change in elevation, metal grate, etc. A few of our participants’ comments recorded in Appendix G may reflect their perception of some detectable warnings as potential walking hazards. Further behavioral testing is needed to assess pedestrians’ willingness to step on detectable warnings with different colors and patterns.

Although there are several factors which can influence whether a pedestrian with low vision will see a detectable warning surface, among the most important are the size (distance) of the warning surface and the luminance contrast of the warning surface with adjacent surfaces. In this study participants were directed where to look to see detectable warnings and were always provided an unobstructed view without any environmental distractions. In real life, other mental demands and correct expectations about where to look and what to expect to see will also influence visual detection. Making all detectable warning installations as similar as possible (location, size, color) may help to match detectable warning characteristics to user expectations.

The image size of detectable warnings (in terms of visual angle subtended) increases as the pedestrian moves closer. For pedestrians with moderate or severe visual impairments, including low visual acuity, or substantial visual field loss, features of the environment which are smaller than several degrees of visual angle may not be detected. High contrast patterns used on detectable warnings in this study were helpful in maintaining high rates of detection on all sidewalk types tested. On the other hand, the pattern elements used may have been too small to be helpful for a few of the participants when they were 7.92 m (26 ft) away. If patterns are used on detectable warnings they should be at least as large as the four inch wide patterns used in this study.

Luminance contrast is important for predicting the number of pedestrians who will be able to see a detectable warning. If particular visibility problems are identified for uniformly colored detectable warning surfaces used adjacent to common paving materials, there may be a need to consider conspicuity enhancements for detectable warnings. Despite some results obtained indoors under artificial lighting which suggested that painted patterns can enhance the visibility of detectable warnings,16 we are aware of no other research that has examined how contrasting visual patterns within the detectable warning surface itself may increase visual detection. Also, we are not aware of any published research which has reported on enhancements to the visibility of detectable warning surfaces through modifications to the surrounding surface (such as painting a dark border around a yellow detectable warning surface to enhance its visibility against light colored concrete).

Visual contrast provided under a standard set of measurement conditions may change as a function of several environmental variables. For detectable warning surfaces installed outdoors, lighting conditions may change drastically throughout the course of the day (and night). Although this study found no effect of illumination level or cloud cover on the detection or conspicuity of detectable warnings, there is a need to determine how the visibility of detectable warning surfaces changes with more extreme changes in natural illumination and with various types and levels of artificial illumination (e.g., street lights). Nighttime illumination (from artificial sources) will have different spectral properties than daylight illumination and may result in less visual contrast between detectable warning surfaces and adjacent surfaces. Further research may be needed to confirm visibility of detectable warnings under low light levels and artificial illuminants.

Dry materials reflect light differently than wet materials and the luminance contrast and color contrast between the detectable warning and its surrounding surface may change when one or both surfaces are wet. Further research may be needed to confirm visibility of detectable warnings under wet conditions.

A final concern is that the colors of detectable warnings and sidewalks can change as the materials age. A particularly striking example of these changes has been related by Kirk:

[The detectable warning product] exhibited considerable fading over the two-year period.... Thus, while the product did not retain its original color, the contrast with adjacent surfaces was increased. This fading of the color, combined with aging of the concrete, actually produced a reversal of the contrast between the detectable warning and the surrounding concrete surface over the two years. When new, the detectable warning was a darker color surrounded by the relatively lighter new concrete; and after two years the detectable warning was a lighter color surrounded by a relatively darker concrete surface.17

The implication of this description is that, as the materials slowly changed color, causing the contrast reversal, there must have been a period of time when the luminance contrast was nearly zero between this detectable warning and sidewalk. Thus, in choosing the detectable warning color for a particular installation, the aging of the materials should be considered.

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 factor 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 4 inches 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.