June 30, 2007
Authors:
James Jenness & Jeremiah SingerPrepared for:
Federal Highway Administration
Washington, DC Prepared by:Prepared by:
Westat
Rockville, Maryland
Background
Detectable warnings are walking surfaces that are principally intended to provide a tactile cue underfoot to pedestrians who have vision impairments. They are installed at potentially hazardous locations such as the edge of a train platform or at the transition between the sidewalk and the street to indicate the change from pedestrian to vehicular use so that a pedestrian traveling without visual cues will not inadvertently step into the road- or rail-way. The tactile properties of detectable warnings result from a grid of small truncated (flat-topped) domes raised above the walking surface. Standardized by the U.S. Access Board, this pattern has proved in research to be detected underfoot or by cane without causing a tripping hazard or obstructing wheelchairs. Detectable warnings that provide salient visual cues in addition to tactile cues can provide additional cues about the location of edges and curb ramps from a greater distance than is possible using tactile cues alone. Some pedestrians use the visual cues provided by the detectable warning to orient to a curb cut or ramp at the end of a crosswalk. Pedestrians with acute vision who are distracted from the primary walking task or otherwise impaired (e.g., cognitively impaired) and children may also benefit from the visual warning if it draws their attention and reinforces the potential hazard of walking beyond the detectable warning.
The diversity of visual abilities among pedestrians makes it difficult to specify optimal visual properties for detectable warnings. Most people who are legally blind have some functional vision, but visual abilities can vary from day to day and vision may be more or less useful depending upon the lighting level and weather. Congenital and acquired physical conditions as well as natural aging processes affect vision in different ways. Individuals may have reduced sensitivity to light/dark differences across a scene (contrast), reduced ability to see certain color differences, complete or relative loss of vision in specific directions (visual field loss), or loss of ability to see fine details (acuity). Despite this diversity of visual abilities, many people with visual impairments will see detectable warnings if there is adequate illumination (ambient lighting level) and adequate luminance contrast (difference in light reflected from a detectable warning and the adjacent sidewalk). Detectable warning color can also help to improve conspicuity and convey meaning.
The ADA Accessibility Guidelines (ADAAG 1991) require that detectable warnings ‘…shall contrast visually with adjoining surfaces, either light-on-dark or dark-on-light’. Draft Public Rights-of-Way Accessibility Guidelines (PROWAG 2005) have a similar provision: ‘Detectable warning surfaces shall contrast visually with adjacent gutter, street or highway, or walkway surfaces, either light-on-dark or dark-on-light.’ Specifiers and agencies sought additional guidance on determining an effective contrast.
FHWA Detectable Warning Study
In 2005, a study was conducted to determine which detectable warning colors and patterns are visually detectable and conspicuous to pedestrians with visual impairments and to provide recommendations related to color, pattern, and luminance contrast of detectable warnings. Fifty individuals with visual impairments participated in separate sessions. Most participants were legally blind (though all had some useful vision) and all reported having some difficulty locating the boundary between sidewalks and streets. The study included men and women ranging in age from 24 to 92 with diverse causes of their visual impairments. The set of 13 detectable warnings (each 3 feet wide by 2 feet deep) tested included 10 solid colors (Figure 1) and three black-and-white patterns including black with white border, black-and-white stripes, and white with black border (Figure 2). Four sidewalk sections (each 4 feet by 8 feet) were fabricated to simulate the appearance of a red brick sidewalk, a dark gray asphalt sidewalk, a white concrete sidewalk, and a brown concrete sidewalk. Each combination of detectable warning color and sidewalk color was viewed outdoors under natural illumination by each of the study participants. Figure 3 shows how the testing area was arranged. Measurements included maximum visual detection distance, self-reported conspicuity rating, and self-reported description of the detectable warning color.
Figure 1. Photographic samples of the 10 single-color detectable warnings tested in the FHWA study
Figure 2. Photographs of three black-and-white patterned detectable warnings on red bricks
Black with White Border
(4-inch wide border)Black/White Stripes
(4.5-inch wide white stripes)White with Black Border
(4-inch wide border)
Figure 3. Plan view of outdoor testing area where participants viewed Detectable warnings on four simulated sidewalk sections
Based on the results of the study, including visual detection rates at 8 feet and 26 feet, conspicuity ratings, and participants’ subjective comments, the following detectable warning colors are recommended for use on asphalt, red brick, and concrete walking surfaces (Table 1). It should be noted that although black-and-white patterned detectable warnings performed very well under the limited conditions tested, additional research is needed to determine if detectable warnings with light/dark patterns (rather than single color detectable warnings) tend to cause visual disorientation or confusion for some people when walking over the surface, particularly when such patterns are installed on a slope.
Table 1. Detectable warning contrast effectiveness assessments based upon FHWA study data
Detectable Warning Color1 |
Surface material surrounding detectable warning |
||
---|---|---|---|
Asphalt |
Brick (Red) |
Concrete |
|
Bright white [e.g., FS 37875] |
Good |
Good |
Not recommended for light colored concrete2 |
White (beige sand texture) |
Good |
OK |
Not recommended2 |
Pale yellow [e.g., FS 23594] |
Good |
Good |
OK |
Federal yellow [e.g., FS 33538] |
Very good |
Very good |
Good |
Light gray [e.g., FS 26280] |
Not recommended |
Not recommended |
Not recommended |
Brown (beige sand texture) |
Not recommended |
Not recommended |
OK |
Orange-red [e.g., FS 22144] |
OK |
Not recommended |
Good |
Bright red [e.g., FS 31120] |
Good |
OK |
Very good |
Dark gray [e.g., FS 36118] |
Not recommended |
Not recommended |
OK |
Black [e.g., FS 17038] |
Not recommended |
Not recommended (may be perceived as a hole) |
Not recommended (may be perceived as a hole) |
Black (or dark gray) and white pattern |
Very good3 |
Very good3 |
Very good3 |
Black (or dark gray) and yellow pattern |
Not tested, but likely to be very good3 |
Not tested, but likely to be very good3 |
Not tested, but likely to be very good3 |
Recommended Minimum Luminance Contrast Values for Detectable Warnings
In general, a minimum luminance contrast of 50 percent is recommended. Lower levels of luminance contrast, however, may be effective if there is a pronounced color difference (i.e. hue, saturation) between the detectable warning material and the adjacent walking surface. For example, if the detectable warning has a highly saturated red or yellow appearance, then installations with luminance contrast as low as 20 percent may be effective for most pedestrians. Adequate luminance contrast also may be achieved with detectable warnings that provide internally contrasting elements (such as from a two-color stripe pattern). In this case the pattern elements should be large (i.e., stripes that are a minimum of 4 inches wide) and should provide a minimum of 60 percent luminance contrast.
Simple Procedure for Making Field Measurements of Luminance Contrast for Detectable Warnings
Photometric field measurements of luminance contrast between an installed detectable warning (or sample of detectable warning material) and the adjacent sidewalk may be made with a luminance meter (such as Minolta LS-100) with a one degree measurement field.
Full Report
The complete research report, Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments, is posted to the Access Board’s website at www.access-board.gov/research/detectable warning-fhwa/report.htm. The report also includes appendices that provide detailed information on visual impairments and their prevalence in the U.S.; the current state of Federal regulations and guidance on detectable warnings; a review of previous research on the visual detection of detectable warnings; a summary of the methods used to measure reflectance, luminance contrast, and chromaticity; a description of the vision tests used to assess the visual capabilities of study participants; a summary of color names used by participants to describe each detectable warning; and a list of unsolicited comments provided by participants regarding the detectable warnings.
Other Useful Resources