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

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.