Appendix C: Previous Research on the Visibility of Detectable Warning Surfaces

Templer, Wineman, and Zimring (1982)
Four participants with low vision completed a study on visual detection of detectable warning surfaces. The goal of this study was to identify painted marking patterns that might be useful to increase the visibility of detectable warnings.

On the first day of the study, the participants, using a long cane as a travel aid, walked over an indoor test course (brushed concrete surface) which incorporated a series of unpainted, 1.1 m (42 in.) x 1.1 m (42 in.) detectable warning surfaces. They were instructed to stop as soon as they detected each of the warning surfaces, and the distance from their forward foot to the warning surface was measured. The warning surfaces were approached from different distances between 2.1 m (7 ft.) and 7.5 m (24.5 ft.) Following the first day of testing, the detectable warning surfaces were painted with various colors and patterns. The paint colors used were red, orange, yellow, yellow-green, green, black, and white. Two different colors were used to create either a repeated stripe, or circle pattern on the warning surface. For all test panels, a .3 m (12 in.) wide band the color of asphalt was painted onto adjoining surfaces to simulate the effect of the warning surface abutting a roadway.

The participants returned on a second day and completed two more walks through the course (with painted warnings). The mean stopping distance on the second day was significantly further from the warnings, by approximately .41 m (16 in.), than the mean stopping distance on the first day, which indicates that the enhanced visual properties due to the paint patterns were effective in increasing detection of the warning surfaces. No significant differences were found between any of the different colors or patterns painted on the warnings. From the methods described, it isn’t clear whether the increase in visual detections of the painted warnings was due to the paint treatment on the panels themselves or to the (presumably dark) asphalt colored paint applied immediately adjacent to the warning surfaces. It is also possible that participants benefited on the second day from practice. No photometric measurements of any of the painted surfaces were reported.

Bentzen, Nolin, and Easton (1994)
This research assessed the visibility of ten pairs of detectable warning surfaces and platform surfaces for 24 persons with visual impairments. Participants were selected based on the following functional vision criteria (self-reported): They had sufficient vision to enable them to tell where a bright light was coming from (light projection); they were unable, or rarely able to read signs, even under optimal conditions; they were unable to reliably see platform edges in interior transit stations; and they were unable to reliably see where curb ramps end and streets begin.

The study was conducted indoors with artificial illumination of approximately 215 lux (20 fc) (it varied along the testing platform from 135 lux (12.5 fc) to 264 lux (24.5 fc). Fluorescent light sources were used. The three platform surfaces used were brushed concrete, coarse aggregate, and black Pirelli tile. Six different detectable warning materials were used. Five of these materials were described as having a yellowish appearance and one was described as grayish red. The color appearance of the warning materials under the conditions of the experiment was further specified by having two observers with normal color vision match each of the surfaces with a set of standard Pantone color chips under the illumination used in the study.

Among the ten pairs of platform surface and detectable warning surfaces, there was one combination for which participants stood on a relatively lighter platform surface and attempted to see a darker detectable warning (dark-on-light contrast). For the other nine pairs tested, participants stood on a relatively darker platform surface and attempted to see a lighter detectable warning (light-on-dark contrast).

The detectable warning surfaces were surrounded on three sides by a cardboard frame. This frame had a 2 ft. x 2 ft. cut out area which, on different trials, either revealed either a 2 ft. x 2 ft. square area of warning surface or it revealed a fabric foil. The fabric foil approximately matched the color of the adjoining platform surface. The participants’ task on each trial was to say whether they saw the detectable warning surface or not. On half of the trials, the participants viewed the warning surface, and on the other half of the trials they viewed a foil (covered warning surface). Participants viewed the detectable warning surfaces from distances of 1.22 m (4 ft) and 2.44 m (8 ft).

The reported performance measure was frequency of correct identification of the presence of a warning surface). Subjective measures included each participant’s choice of the three most visually detectable warning and surround pairs and the most detectable pair, as well as the least detectable pair.

There were no significant differences among any of the warning and surround pairs which had contrasts of 40 percent or greater. All of these were detected at rates greater than 90 percent which were not significantly different than 100 percent. Only the lowest contrast (25%) pair of brushed concrete platform and dark orange yellow (Pantone 141c) detectable warning was detected at a significantly less frequently than the others, at a rate of 86 percent.

The participants’ subjective judgments of which pairs had the “best visual contrast” showed a preference for federal yellow (Pantone 109u) warning on the coarse aggregate platform (62% contrast) or for the same warning surface on the brushed concrete platform (40% contrast). Other preferred pairs were Pantone Process yellow u warning on the coarse aggregate platform (70% contrast), and grayish red (Pantone 187u) warning on the brushed concrete platform (50% contrast, platform lighter), and light yellowish brown (Pantone 1245u) warning on the black Pirelli tile (75% contrast).

The two pairs chosen by the greatest number of participants to have the “worst” visual contrast, defined as “a surface participants would not like to see put down on the edge of a transit platform because it is either undetectable or unreliably detectable” were the grayish red (Pantone 187u) warning paired with the black Pirelli tile platform (67% contrast) and the dark orange yellow (Pantone 141c) warning paired with brushed concrete (25% contrast).

The authors emphasized that measured luminance contrast was not predictive of participants’ preferences for “best” and “worst” visual contrast, that federal yellow may be a good color to choose for a standardized warning surface, and that it may be important to ensure that the lighter surface in a contrast pair has sufficiently high reflectance.

Virginia Department of Transportation (O’Leary, Lockwood, & Taylor, 1996)
A group of 27 partially sighted pedestrians participated in this study of detectable warnings which was conducted outdoors on the grounds of the Virginia Rehabilitation Center for the Blind (Richmond, VA). Participants were characterized by their normal use of travel aids: 37 percent used no travel aids, 48 percent used canes, 4 percent used guide dogs, 18 percent used sighted guides and some used multiple travel aids. No information was given about time of day or lighting conditions under which the tests were conducted.

Seven different detectable warning materials were installed at various intervals along an existing straight, flat concrete sidewalk which bisected a grassy area. Each of the detectable warnings was 1.22 m (4 ft) deep and extended across the full 8-foot width of the sidewalk.

The seven surface materials tested included:

1) Precast exposed aggregate conforming to VDOT standards (No. 57 river gravel and natural sand).
2) Precast exposed aggregate using a smaller graduation of gravel (No. 7) and manufactured sand.
3) Precast dark gray (black) concrete with raised truncated domes.
4) Precast concrete with lateral raised corduroy pattern running parallel to the direction of travel.
5) Red pavers with raised truncated domes.
6) Yellow rubber Pathfinder tiles with raised truncated domes.
7) Yellow composite Pathfinder tiles with raised truncated domes.

Note that only four of the surfaces tested had raised truncated domes conforming to ADAAG.

The surfaces that provided more color contrast were detectable from farther away than the two aggregate surfaces which were rated as hard or very hard to detect. The authors also noted that, “Although the corduroy [surface] did not provide much color contrast, partially sighted individuals readily saw the distinctive ribbed pattern (74 percent said they detected the corduroy surface by sight).” Thus, it is possible that the colored truncated domes used in this study were more visually detectable than course aggregate surfaces due to their raised surface characteristics in addition to differences in color and luminance contrast with the surround.

Table C1 below has been modified from data reported by O’Leary, Lockwood, and Taylor (1996) in their Table 1. The table shows the percentage (and cumulative percentage) of participants who first detected the warning surface at the distance given in the left column. Although it is not certain whether detection was accomplished through vision at distances less than 2.5 m (8.2 ft), it is very likely that vision was used to detect the warnings at distances greater than 2.5 m, therefore the cumulative percentages shown in bold type represent the percentage of participants who detected each surface visually. Note that although the yellow composite domes were detected visually by 63 percent of the participants, this surface was never detected at a distance greater than 5 m (16.4 ft). This is surprising, given that approximately 41 percent of the same participants detected yellow rubber domes at distances greater than 7.5 m (24.6 ft).

Table C1. Percentages and Cumulative Percentages of Visually Impaired Participants Who Detected Warning Surfaces from Various Distances (data from O’Leary, Lockwood, & Taylor, 1996)

Detection Distance

State Std. Exposed Aggregate

Small Graduation Exposed Aggregate

Concrete Corduroy Surface

Red Pavers With Domes

Yellow Rubber Domes

Yellow Composite Domes

Black Concrete Domes

>15m

14.8 (14.8)

14.8 (14.8)

22.2 (22.2)

0.0 (0.0)

3.7 (3.7)

0.0 (0.0)

25.9 (25.9)

10 – 14.99m

7.4 (22.2)

7.4 (22.2)

3.7 (25.9)

25.9 (25.9)

0.0 (3.7)

0.0 (0.0)

14.8 (40.7)

7.5 – 9.99m

0.0 (22.2)

3.7 (25.9)

3.7 (29.6)

3.7 (29.6)

37.0 (40.7)

0.0 (0.0)

0.0 (40.7)

5 – 7.49m

3.7 (25.9)

7.4 (33.3)

11.1 (40.7)

3.7 (33.3)

0.0 (40.7)

0.0 (0.0)

3.7 (44.4)

2.5 – 4.99m

7.4 (33.3)

0.0 (33.3)

11.1 (51.8)

11.1 (44.4)

11.1 (51.8)

63.0 (63.0)

14.8 (59.2)

Less than 2.5m or on surface

37.0 (70.3)

48.1 (81.4)

44.4 (96.2)

55.5 (100)

48.1 (100)

37.0 (100)

37.0 (96.2)

Did not detect

29.6

18.5

3.7

0.0

0.0

0.0

3.7

Sacramento Transit District (Bentzen & Myers, 1997)
A focus group on the visual detectability of warning surfaces by persons with low vision was conducted as part of a product evaluation for the Sacramento Transit District (Bentzen & Myers, 1997).

Six participants, ages 33 to 58, were recruited who had vision sufficient to see high contrasts, but insufficient to see RT tracks (light rail) under most lighting conditions. They all used the Sacramento RT system at least 1-5 times per week. The focus group participants evaluated four different installed detectable warning materials by viewing them while walking back and forth over them, viewing them from distances of 6.1 m (20 ft), 9.1 m (30 ft), and 12.2 m (40 ft), and then by viewing samples of the same four materials at close range in a well lit conference room. The tests were conducted outdoors on one day in December in the late afternoon (4:30 PM). The testing took place under an “unexpectedly low level of light” due to weather conditions of “light drizzle, increasing to rain during the testing session.” The testing platform was illuminated and wet, and the authors noted that, “In addition to the visual contrast attributable to differences in color, the warning surfaces differed in light reflectance attributable to glare off their wet surfaces. The particular materials evaluated were 6 m (20 ft) lengths (.6 m (24 in.) wide) of:

1) Armor Tile (federal yellow)
2) High Quality (federal yellow)
3) Interlock (bright yellow, “significantly duller” than the other three manufacturers’ federal yellow)
4) Detectable Warning Systems (federal yellow)

The material surrounding the detectable warning surfaces was pavers rather than concrete, but the color of the pavers was not reported. No measurements of luminance contrast or illumination were reported for this study.

Participants rated the Armor Tile and High Quality products as providing significantly greater visual contrast than the other two products, while the Detectable Warning Systems product was rated significantly higher then the Interlock product. The mean maximum distance at which the surfaces could be seen was approximately 6.7 m (22 ft) for the Interlock product. This was significantly less than the mean distances of 9.1 – 10 m (30 – 33 ft) for the other products, which did not differ significantly from each other.

These results indicate that some pedestrians whose self-reported visual function may not be sufficient to reliably detect a hazard (such as light rail tracks) may be able to see federal yellow detectable warnings from distances of at least 6.1 – 9.1 m (20 - 30 ft). If approaching the warning surfaces on foot, these pedestrians would likely be able to see the detectable warnings well before they stepped on them. Also, visibility of detectable warnings at distances of 6.1 – 9.1 m (20 - 30 ft) may be useful for guiding pedestrians to the destination curb while crossing a street.

Wisconsin Department of Transportation (Kemp, 2003)
A series of product trials of several detectable warning surface materials were conducted to assess ease of installation, durability, and other properties. Two color trials (informal assessments of visibility) were conducted, although few details concerning the methods are given in the report. Apparently, one person with a visual impairment participated in both trials, and one additional person participated in the second trial. The first participant had a reported visual acuity of 20/200. The second participant had visual function sufficiently poor that he was only able to detect one color sample (black) from a distance of 1.5 m (5 ft).

In the first trial, the participant viewed 22 samples of masonry blocks of various colors, a federal yellow tile and a black tile. No other details are given, except that the evaluation was done outside on a bright day. The yellow tile was distinguishable from 10 m (33 ft), but red masonry samples were distinguishable at 5.5 m (18 ft). It was noted that a gloss finish on the masonry samples made them easier to distinguish.

In the second trial, eight different detectable warning surface color samples (“Armortile,” Engineering Plastics) were evaluated by two people with visual impairments. The colors of the tiles were blue, rust, federal yellow, white, black, light gray, dark gray, and bright yellow. Maximum recognition distances were measured for each sample, and subjective ratings of contrast (4 point scale) were obtained. The second participant was unable to visually detect any of the tiles except for the black tile. The first participant recognized the white and federal yellow tiles from a distance of approximately 14 m (46 ft). The bright yellow tile was recognized at 8.5 (28 ft), the dark gray, black, rust, and blue tiles were recognized at 3 – 4.9 m (10-16 ft), and the light gray was not seen. The participant rated the white tile (rating = 1) as having the greatest contrast, followed by the bright yellow (2) and federal yellow (2), then rust (3) and blue (4). The other colors were not rated because they provided insufficient contrast.