Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments

Detectable warnings are walking surfaces that are primarily intended to provide a tactile cue to pedestrians who are visually impaired. They are installed at locations such as the edge of a train platform or at the transition between the sidewalk and the street to warn pedestrians of the potential hazard that lies ahead. The tactile properties of detectable warnings result from a grid of small, truncated (flat-topped) domes across the warning surface. This pattern has been standardized by the U.S. Access Board and testing has shown that the pattern can be detected underfoot or by cane without causing a tripping hazard or obstructing wheelchairs. Despite the proven tactile benefits of detectable warnings, little research has been conducted to evaluate the visual detectability of various detectable warning materials. Detectable warnings that provide salient visual cues in addition to tactile cues may help many pedestrians with visual impairments to locate hazards or curb ramps from a greater distance than is possible using the tactile cues alone. Some pedestrians may use them to orient to a curb cut or ramp at the end of a crosswalk.

The objectives of this study were (1) to determine which detectable warning colors and patterns are visually detectable and conspicuous to pedestrians with visual impairments and (2) to provide recommendations related to color, pattern, and luminance contrast of detectable warnings for placement on sidewalks.

Fifty men and women ranging in age from 24 to 92 participated in this study. All participants had impaired but useful vision. Most were legally blind. All participants reported they had difficulty locating the boundary between sidewalks and streets.

Thirteen detectable warnings were tested. The set included ten uniform colors (white, simulated white concrete, simulated brown concrete, light gray, dark gray, bright federal yellow, pale yellow, bright red, orange-red, and black) and three black-and-white patterns. Each detectable warning was a .91 m (3 ft) wide by .61 m (2 ft) long composite panel designed for surface application. Participants viewed each detectable warning on four different horizontal backgrounds. Each background was 1.22 m (4 ft) wide by 2.44 m (8 ft) long and was constructed to simulate the appearance of a red brick sidewalk, a dark gray asphalt sidewalk, a white concrete sidewalk, and a brown concrete sidewalk. The study was conducted during midday hours with dry surfaces.

Participants viewed each combination of detectable warning and sidewalk individually. To determine detection distance, participants first viewed the sidewalk from 7.92 m (26 ft) away and, if they could not see the detectable warning from this distance, they began to walk closer until they were confident that a detectable warning was present. On some trials there was no detectable warning present. Once detectable distance had been measured, participants were asked to view the detectable warning from a distance of 2.44 m (8 ft) and to describe the color and/or pattern of the detectable warning. Finally, participants were asked to rate the conspicuity (attention-getting property) of the detectable warning on a five-point scale.

Participants viewed each combination of detectable warning and background color individually. To determine detection distance, participants first viewed the simulated sidewalk section from 7.92 m (26 ft) away and if they could not see the detectable warning from this distance, they began to walk closer until they were confident that a detectable warning was present. On some trials there was no detectable warning present. After detection distance had been measured, participants viewed the detectable warning from a distance of 2.44 m (8 ft) and described its color, and rated the conspicuity (attention-getting property) of the detectable warning on a five-point scale.

Detection distance results indicate that pedestrians with visual impairments were able to see most combinations of detectable warning and sidewalk from 2.44 m (8 ft) away, but were less likely to see them from 7.92 m (26 ft) away. Detectable warnings that were similar in color to the sidewalk were seen by few participants, indicating that visual cues provided by the truncated dome pattern itself are not sufficient to ensure visual detection. The color of the sidewalk strongly influenced how easily single-color detectable warnings could be seen; however, black-and-white patterned detectable warnings were visually detectable and conspicuous for most participants across all sidewalk types. The luminance contrast provided by the detectable warning and the sidewalk (or by the patterns) was an important factor for predicting the likelihood that a detectable warning would be seen. Where luminance contrast was 70 percent or greater, about 95 percent of participants were able to see the detectable warning from 2.44 m (8 ft) away. Detectable warnings that provided at least 60 percent contrast could be seen by about 92 percent of participants from 2.44 m (8 ft) away. Dark detectable warnings on a dark sidewalk were an exception. Although providing moderately high luminance contrast, these combinations were detected less often than would be predicted from their luminance contrast.

Detection distance results indicate that pedestrians with visual impairments were able to see most combinations of detectable warning and sidewalk from 2.44 m (8 ft) away, but were less likely to see them from 7.92 m (26 ft) away. Detectable warnings that were similar in color to the sidewalk were seen by few participants, indicating that visual cues provided by the truncated dome pattern itself are not sufficient to ensure visual detection. The color of the sidewalk strongly influenced how easily single-color detectable warnings could be seen, however, black-and-white patterned detectable warnings were visually detectable and conspicuous for most participants on all sidewalk colors tested. The luminance contrast provided by the detectable warning and the sidewalk (or by the patterns) was an important factor for predicting the likelihood that a detectable warning would be seen. Where luminance contrast was 70 percent or greater, about 95 percent of participants were able to see the detectable warning from 2.44 m (8 ft) away. Detectable warnings that provided at least 60 percent contrast could be seen by about 92 percent of participants from 2.44 m (8 ft) away. Dark detectable warnings on a dark sidewalk were an exception. Although providing moderately high luminance contrast, these combinations were detected less often than would be predicted from their luminance contrast. For dark single-color detectable warnings and black-and-white patterned detectable warnings a few participants commented that the detectable warning looked like something else (e.g. hole, metal grate).

Besides luminance contrast, regression analyses indicated that some other characteristics of detectable warnings were generally associated with high detection rates and high conspicuity ratings. These include color (reds and yellows rather than achromatic) and reflectance (lighter colors rather than darker colors). For the range of conditions tested, neither illumination level (per trial) nor sky conditions (percent cloud cover per session) affected detection and conspicuity of detectable warnings.

Further visibility testing of detectable warnings should include a broader range of lighting conditions (dusk, dawn, artificial illumination), determination of optimal internal contrast patterns for two-color detectable warnings, and viewing detectable warnings in naturalistic roadway environments with unpredictable crossing locations, distractions, visual obstructions, wet surfaces, and so forth. Further research also should include pedestrians’ perceptions of different detectable warning colors (e.g. Is the detectable warning recognized as being safe to step on? Does the detectable warning convey the intended message?).

1.1 Objectives
The primary objectives of this project are to determine whether various detectable warning materials are visually detectable by pedestrians who have visual impairments and to provide recommendations related to color and luminance contrast of detectable warnings.

1.2 Background
Detectable warning surfaces are intended primarily to provide a tactile cue to pedestrians who are blind or have visual impairments. Major causes of visual impairments in the United States are described briefly in Appendix A. The majority (80%) of people who are legally blind retain some degree of visual function1, and these people, along with pedestrians who have less severe visual impairments, may benefit from detectable warnings, which are both visually and tactilely distinctive.

The tactile properties of detectable warning surfaces result from a grid pattern of raised, flat-topped, truncated domes that can be felt underfoot or detected by a long cane or a wheelchair without causing a tripping hazard. The size and spacing of the truncated domes have been clearly specified by the U.S. Access Board.2 3 However, guidance concerning the visual properties of detectable warning surfaces is much less specific. The U.S. Access Board states that “Detectable warning surfaces shall contrast visually with adjoining surfaces, either light-on-dark or dark-on-light.”4 The Public Rights-of-Way Access Advisory Committee has previously noted that there is a lack of human factors research with vision-impaired pedestrians.5 There is not a sufficient quantitative research basis to support any more specific guidance with respect to the visual properties of detectable warning surfaces, particularly color and contrast. An overview of Federal rule making and guidance on detectable warnings is given in Appendix B.

In this report, the terms “detectable warning” and “detectable warning surface” refer to the standard truncated dome surfaces described in the Americans with Disabilities Act Accessibility Guidelines (ADAAG) and described by the U.S. Access Board.6 7 8 9 10 Note that detectable warnings with truncated domes as used in the U.S. are only one of several types of tactile patterns used through the world as detectable warnings, and represent only a subset of the types of Tactile Ground Surface Indicators (TGSIs) that are being used as a navigational aid for pedestrians who are visually impaired. For example, various tactile pavements have been used in Japan since the 1960s, and in England there are currently seven different types of tactile paving patterns used.11 12 13 Persons interested in the practices of other countries may wish to consult Detectable Warnings: Synthesis of U.S. and International Practice, which is available from the U.S. Access Board.14

1.3.1 Need for Detectable Warnings at Curb Ramps
For pedestrians who have visual impairments, curbs used to be a reliable cue for detecting the boundary between the sidewalk and a street. However, now that curb ramps and other flush transitions are used at crosswalks to improve the accessibility of sidewalks for people who cannot negotiate curbs, the curb edge has become a less reliable navigational cue for many pedestrians. This is especially relevant for blind pedestrians traveling in unfamiliar areas. Bentzen and Barlow (1995) reported that blind pedestrians using a long cane failed to detect the edge of an intersecting street on 39% of 557 approaches to unfamiliar intersections, and that shallower ramps were more difficult to detect than steeper ramps. For curb ramps encountered with slopes of 4 degrees (1:14) or less, there was a 51% rate of entering the street rather than stopping on the sidewalk or ramp.

1.3.2 Detectable Warnings May Provide Visual Guidance
Pedestrians who are visually challenged often have difficulty locating crosswalks, properly aligning themselves to cross, determining when it is safe to cross, maintaining a straight path while crossing, and completing their crossing before perpendicular traffic approaches.15 For pedestrians who have some functional vision, detectable warnings that can be seen before they are detected by cane or under foot may provide useful information:

1.3.3 Visual Detection of Detectable Warning Surfaces
A search of the literature found reports on installation and durability of detectable warning surfaces (e.g., Ketola & Chia, 1994; Kaplan, 2004) and some studies that have evaluated detectable warning surfaces for detection under foot or by long cane (e.g., Peck & Bentzen, 1987; Bentzen, Nolin, Easton, Desmarais, & Mitchell, 1994; Tijerina, Jackson, & Tornow, 1994, 1995). In general, the participants selected for these detection studies have had little or no functional vision (usually light perception only) so that tactile and auditory detection could be evaluated without having the results confounded by visual detection. Only five reports were found which included visual assessments of detectable warning surfaces by people with visual impairments (Templer, Wineman, & Zimring, 1982; Bentzen, Nolin, & Easton, 1994; O’Leary, Lockwood, & Taylor, 1996; Bentzen & Myers, 1997; Kemp, 2003). Each of these reports is described in detail in Appendix C.

These studies varied widely in terms of the number of participants, types and number of detectable warning materials tested, procedures used, and the amount of detail provided in the report. Participants in these studies generally were recruited based on self reported visual ability. Three of the studies used six or fewer participants, and the other two studies had 24 and 27 participants. One of these was conducted outdoors under natural illumination, and the other was conducted indoors under artificial illumination. The size of the detectable warnings used varied widely between studies. Most of the studies reviewed do not adequately report on the lighting conditions and color and luminance contrast between detectable warning surfaces and surrounding surfaces. Clearly, some of the studies were meant only to be informal assessments of particular products rather than scientifically rigorous experiments.

Overall, yellow detectable warning surfaces (particularly federal yellow, also known as safety yellow) have been found to be highly visually detectable and, as expected, higher contrasts between the warning surface and the adjacent surface are more detectable than lower contrasts. Participants have generally rated federal yellow warning surfaces as being highly detectable. Although dark-on-light contrast pairs have not been tested as often as light-on-dark contrast pairs, there is some indication that they may be just as effective. Finally, there is some evidence that low reflectance of the lighter surface in a contrast pair may reduce visibility, even when luminance contrast is moderately high.

Systematic outdoor evaluations were performed on 13 different detectable warning colors/patterns on 4 different simulated sidewalk surfaces by 50 visually impaired participants who have visual impairments. The two main dependent measures were:

1.4 Participants
Fifty adults with low vision were recruited through contacts with local organizations for people who are blind or visually impaired. Information about the study was also distributed through email lists, flyers in medical offices and retirement communities, and through personal contacts of orientation and mobility specialists in the Washington, DC, area. Some participants were referred by other participants. Individuals were invited to participate based on their responses to a screener questionnaire. When people called to inquire about the study they were asked about the nature and severity of their visual impairments, their frequency of travel and difficulties experienced while walking, and the travel aids they use. Self-reported difficulty in detecting streets and curb ramps was a major criterion used to select participants for the study. Participants were compensated with $75 for their time and were reimbursed for local travel expenses.

Participants ranged in age from 24 to 92 years old, with a median age of 54. The distribution of participants’ ages by gender is shown in Figure 1. There were 31 women and 19 men in the sample. Each age group contained both men and women except for the youngest age group (20 to 29) and oldest age group (90 to 99) which each consisted of women only and the 70 to 79 years-old group which consisted of men only. The most frequently reported travel aid was a long cane, which was used by 36 participants. Some participants reported that they use more than one kind of travel aid, choosing what they need based on the duration of the planned trip, their familiarity with the area where they will be traveling, and the anticipated lighting conditions. The use of travel aids during the study was permitted. Mobility aids such as support canes and walkers were also allowed. Eyeglasses and sunglasses were allowed, though viewing scopes such as monoculars were not allowed.

Nearly all of the selected participants were legally blind as a result of limited visual acuity, limited field of vision, or a combination of the two, but all participants had some useful vision (more than light perception). None of the participants had a driver’s license and all reported walking on sidewalks occasionally or frequently, either with or without travel aids. The number of participants who reported using travel aids at least occasionally is shown in Table 1. The most frequently reported travel aid was a long cane, which was used by 36 participants. Some participants reported that they use more than one kind of travel aid, choosing what they need based on the duration of the planned trip, their familiarity with the area where they will be traveling and the anticipated lighting conditions. The use of travel aids during the study was permitted. Mobility aids such as support canes and walkers were also allowed. Eyeglasses and sunglasses were allowed, though viewing scopes such as monoculars were not allowed.

Participants’ visual impairments were diverse and in several cases vision was affected by multiple medical conditions. The complete list of participants’ self-reported conditions affecting their vision is given below in Table 2. The most commonly reported conditions were glaucoma, cataract, and macular degeneration.

Travel Aid	Participants Reporting Use
Long cane	36
Dog guide	4
Monocular/ telescope/ magnifying glasses	3
Short cane / support cane	6
Walker	2
Wheel chair	1
No travel aids used	10

Medical Condition	Participants Reporting Condition
Glaucoma	13
Cataract	12
Macular degeneration / macular dystrophy	10
Retinitis pigmentosa	6
Optic neuritis / optic nerve atrophy	6
Brain injury	5
Diabetic retinopathy	5
Retinopathy of prematurity	4
Retinal detachment	3
Albinism	2
Corneal dystrophy / other corneal disease	2
Myopic degeneration	1
Inverse retinitis pigmentosa	1
Uveitis	1
Stargardt’s disease	1
Giant cell arteritis	1

1.5 Materials
Sidewalks. Four different colors of simulated sidewalk surfaces were used in this study. These included white (simulating new concrete), brown (simulating aged, dirty concrete), dark gray (simulating asphalt), and red (actual paving bricks). The four simulated sidewalk sections were constructed on low wooden platforms; each covered a 1.22 m (4 ft) wide x 2.44 m (8 ft) long level area on the ground. The white and brown “concrete” surfaces were simulated by applying several coats of paint and sand mixture to a sheet of OSB plywood (oriented strand board). The surface texture provided by the OSB plywood and paint/sand mixture approximated the surface texture of brushed concrete. Dark gray asphalt rolled roofing material was glued on OSB plywood to simulate dark gray asphalt pavement, and red colored concrete paving bricks were laid (without mortar joints) to simulate the brick sidewalk. The surface of each simulated sidewalk section was raised approximately 76 mm (3 in.) above the ground level. Chromaticity and luminous reflectance measurements of the simulated sidewalk surfaces used in this study are given in Table 3.

Detectable Warnings. Thirteen different detectable warnings were tested in this study. Ten were uniformly colored and three others had black-and-white patterns. Although two-color detectable warnings are not commonly used, we included a few high contrast patterns in this study to determine if patterns might be more effective than uniformly colored detectable warnings. Other colored patterns might have been tested but we chose to limit the number of detectable warnings used in the study to 13 so a participant could complete a full set of trials in a single two-hour session. The 10 uniformly colored detectable warnings are shown in Figure 2 with color samples. The three black-and-white patterned detectable warnings are shown in Figure 3, photographed against the red brick sidewalk.

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)

The detectable warnings were surface-mounted composite panels provided by ADA Armor-Tile. The panels were .89 m (35 in.) x .65 m (25.5 in.) including a 13 mm (0.5 in.) smooth tapered edge on all sides. The spacing and size of the truncated domes were compliant with Federal geometric specifications (see Appendix B). The detectable warnings also had very small bumps located between and on the truncated domes to provide texture for traction. Because this study was designed to examine the relative visibility of different detectable warning colors, materials from a single manufacturer were used so that the geometric characteristics of the truncated dome pattern would be constant for all of the different colors tested. Some of the colors selected for inclusion in the study were standard colors provided by the manufacturer, and some of the colors were created by painting the detectable warning panels. The relatively thin surface-mounted detectable warnings were chosen for use in this study because they could be lifted easily by a single experimenter. However, because they were not permanently mounted according to the manufacturer’s specifications, the detectable warnings had a tendency not to lie completely flat on the sidewalk. To overcome this problem, several thin metal plates were glued to the bottom of each detectable warning and magnets were embedded in the sidewalks, flush with the surface. This system served to hold the detectable warning panels flat against the sidewalk but allowed them to be quickly removed and replaced between different trials during the course of the study. When in place, the front edge of each detectable warning was positioned 1.22 m (4 ft) behind the front edge of the sidewalk and was centered horizontally on the sidewalk section.

Blanks. Blank panels were also created for this study. Blanks were made from thin sheets of painted plastic or asphalt roofing material approximately the same size as the detectable warnings. These blanks were the same color as the sidewalks and had no truncated domes. Blanks provided very little visual contrast against the sidewalk and were included in the study to ensure that participants could not simply assume that a detectable warning was present on every trial. Like the detectable warnings, the blanks cast small shadows along their front edges which ensured that participants could not simply use the presence of a thin shadow as a cue to determine that a detectable warning was present on the sidewalk. Blank panels were used with the white, brown, and asphalt sidewalks, but not with the brick sidewalk.

Chromaticity and Reflectance of Materials. Chromaticity and reflectance are physically measurable qualities that are related to the perceived color and lightness of surfaces. The chromaticity and reflectance of detectable warnings, sidewalks, and blank panel surfaces were measured in place (horizontal) at the testing site. Table 3 shows the chromaticity coordinates and reflectance factors of the materials used in this study. This set of chromaticity measurements was made between 10:00 a.m. and 11:00 a.m. under natural illumination (20% cloud cover) using a SpectraScan PR650 (PhotoResearch) spectrophotometer. The reflectance factors were measured on a different day using a Minolta CS-100 Chroma Meter. All measurements were made from the same direction that the surfaces were viewed by participants: at a downward angle of 45 degrees. Other details about the measurement procedures are given in Appendix D. Additional photometric measurements of real sidewalks at various locations in Rockville, Maryland confirmed that our simulated sidewalks had chromaticities and luminance reflectances that are plausible for actual paving materials.

Chromaticity and reflectance of the actual concrete sidewalks measured varied considerably depending on their age and dirtiness. For example, a new-looking, clean concrete sidewalk had a luminance reflectance of r = 0.49, while an older, much darker concrete sidewalk had a luminance reflectance of r = .09. Three asphalt sidewalks measured had luminance reflectance ranging from r = .06 to r = .10. Thus, from our limited measurements of real sidewalks, we conclude that the white simulated concrete sidewalk section used in the present study is similar to very light new concrete, and the simulated asphalt sidewalk is similar to new dark asphalt. The luminance reflectance of the brown simulated sidewalk is consistent with medium gray concrete sidewalks that we observed, although its appearance is more brownish. The red bricks used in the present study had similar luminance reflectance as bricks measured in existing sidewalks.

Material	CIE1931 Chromaticity Coordinates	Reflectance Factor
White detectable warning	x = .333, y = .347	.74
Light Gray detectable warning	x = .326, y = .341	.24
White “concrete” detectable warning	x = .352, y = .364	.64
Brown “concrete” detectable warning	x = .390, y = .386	.17
Dark Gray detectable warning	x = .320, y = .331	.09
Federal Yellow detectable warning	x = .511, y = .454	.46
Pale Yellow detectable warning	x = .412, y = .414	.47
Bright Red detectable warning	x = .587, y = .323	.11
Orange-Red detectable warning	x = .533, y = .356	.13
Black detectable warning	x = .324, y = .338	.02
White paint (used for border & stripe patterns)	x = .330, y = .344	.82
Black paint (used for border pattern)	x = .326, y = .340	.02
Brick sidewalk (for a typical brick)	x = .417, y = .358	.15
Asphalt sidewalk	x = .332, y = .346	.06
Asphalt “blank” panel	x = .335, y = .349	.05
White sidewalk	x = .351, y = .363	.57
White “blank” panel	x = .351, y = .363	.60
Brown sidewalk	x = .385, y = .381	.17
Brown “blank” panel	x = .384, y = .381	.17

1.6 Testing Site and Conditions
The study was conducted on a flat, outdoor patio adjacent to Westat’s conference center in Rockville, MD. The area was clear of obstructions and was situated so that no shadows fell on the four sidewalk sections during the hours when testing was conducted. The four sidewalk sections were arranged side-by-side and a straight, unobstructed walking path was provided to each sidewalk. A scale drawing of the site is presented in Figure 4. Study sessions began between 10:00 a.m. and 1:30 p.m. to ensure consistent lighting. Sessions were conducted regardless of cloud conditions, but were canceled in the event of rain. Testing sessions took place between May and August 2005. When not in use, the sidewalk sections were covered to prevent damage from exposure to sunlight and precipitation, and detectable warning panels were stored indoors.

1.7 Procedure
The study utilized a full factorial, repeated-measures design in which participants viewed each of the 13 detectable warnings against each of the 4 sidewalks for a total of 52 trials. Two “blank” trials were also inserted for each of the sidewalks (except the brick sidewalk) for a grand total of 58 trials. Participants completed all trials on a particular sidewalk before moving on to the next sidewalk. The order in which sidewalks were viewed was randomized for each participant, as was the order of the detectable warnings (and blank trials) viewed on each sidewalk. Participants were tested individually in sessions lasting 1.5 to 2.5 hours.

The purpose and general activities involved in the study were explained to participants during initial telephone screening. Consent forms were printed in a large font size and sent to participants via mail or e-mail (whichever was preferred).

1.7.1 Introduction and Vision Testing
Upon arrival at Westat, participants were guided to the vision testing room. Participants were escorted at all times by an experimenter who had received training from an orientation and mobility specialist. The experimenter first read the consent form aloud to participants who had not had the opportunity to read it themselves and then collected signed consent forms. Next, participants were asked to describe their visual condition, the functionality of their vision, and their use of mobility aids. Three separate vision tests were performed to assess participants’ visual acuity, contrast sensitivity, and color vision. For these tests, participants were allowed to view the charts binocularly and to use head or eye movements necessary to read as many letters or symbols as possible. Details on the vision testing procedures are given in Appendix E. No formal assessments of each participant’s visual fields were conducted due to time constrains, however based on self-reports, the study sample included participants with small, medium, and large visual field losses.

The experimenter guided participants to the outdoor testing site and familiarized them with the site layout. Participants were given a small sample piece of detectable warning material to see and feel, then were shown examples of a detectable warning and a blank on a sidewalk. The experimenter then described the study procedures and guided participants through a practice trial before beginning the study trials.

1.7.2 Visual Detection Distance
Each trial began with the participant standing 7.9 m (26 ft) from the front edge of the detectable warning. The 7.9 m (26 ft) viewing distance was chosen to approximate the width of a residential street. Participants began facing away from the sidewalk to allow a second experimenter to lay down a detectable warning (or blank). When the detectable warning was in place, the participant turned around and reported to the experimenter whether he/she was confident that there was a detectable warning on the target sidewalk. If the detectable warning was not seen from 7.9 m (26 ft) away, the participant was instructed to walk slowly toward the sidewalk and to stop immediately if he/she became confident that there was a detectable warning (and not a blank) present. If the participant came within 2.4 m (8 ft) of the detectable warning and could not confidently say that a detectable warning was present, the trial was ended.

1.7.3 Color Naming, Conspicuity Rating, and Other Comments
If the participant was able to see the detectable warning from at least 2.4 m (8 ft) away, the experimenter guided the participant to the 8-foot line to ask two more questions. First, the participant was asked what color or pattern they saw on the detectable warning. Second, the participant was asked to rate the likelihood that the detectable warning would attract his/her attention on that particular sidewalk (conspicuity). A rating scale of 1 to 5 was used where 1 meant (the detectable warning is very unlikely to attract my attention on this type of sidewalk) and 5 meant (the detectable warning is very likely to attract my attention on this type of sidewalk).

If the participant did not see the detectable warning from 2.4 m (8 ft), a conspicuity rating of zero was assigned by the experimenter. Although additional comments were not solicited, the experimenter also recorded any relevant comments that the participant provided about the detectable warnings, such as, “looks like a cement patch.”

2.1 Participants’ Vision
The distribution of acuity scores for participants in this study is shown in Figure 5. Normal visual acuity on this scale is 20/20, which is not shown because it would be plotted far to the right of the distribution. From the adjusted viewing distance of 1.22 m (4 ft), one participant was able to read the bottom line of the chart (smallest letters) which indicated that her acuity was 20/50 or better. Despite having relatively good acuity, this participant was retained in the study because she reported having a large visual field loss. Forty-two of the participants had visual acuities of 20/200 or less, including nine participants who were unable to read the top line (largest letter) of the acuity chart, indicating that their visual acuity was less than 20/1000.

Participants’ contrast sensitivity was tested with a Pelli-Robson Contrast Sensitivity chart. The letters on this chart are all large, but the contrast of letters decreases from left to right and from the top of the chart to the bottom. Participants viewed this chart from a distance of one meter. The chart was illuminated uniformly according to the instructions provided by the manufacturer. Figure 6 shows the distribution of participants’ contrast sensitivity scores. Note that the common logarithm of contrast sensitivity is plotted along the abscissa. A log contrast sensitivity score of zero indicates that the participant was able to read only the black letters on a white background at the full level of 100 percent contrast. The nominal value for normal log contrast sensitivity is 2.0, which corresponds to the ability to distinguish letters having only 1 percent contrast. All of the participants in this study had reduced contrast sensitivity. There were 15 participants who were unable to read even the highest contrast letters on the Pelli-Robson chart from a distance of one meter. This group is shown by the “N” label on the left side of the abscissa to indicate that their contrast sensitivity was not measurable with this test.

The recently revised (Fourth Edition) of the H.R.R. Pseudoisochromatic Plates test (Richmond Products, Boca Raton, FL) was used to screen participants for red-green and blue-yellow abnormalities in color vision. Because most participants in this study were not able to read any symbols on the H.R.R. Pseudoisochromatic Plates test, no results for this test are reported (see Appendix E for a description of vision testing procedures).

2.2 Lighting Conditions
At the beginning of each testing session, experimenters visually assessed sky conditions and estimated the percent cloud cover. On each trial one measurement of horizontal illuminance was manually recorded from a Minolta T-1 Illuminance meter. The mean illuminance and standard deviation of illuminances for each participant’s trials are plotted in Figure 7. Data have been ordered from left to right by the estimated percent cloud cover that was present during the participant’s testing session. Note that the scale on the abscissa is categorical (not linear) to provide clear separation between data points. This figure shows that the fifty testing sessions varied in terms of percent cloud cover from 0 percent to 100 percent, and that mean illumination for different participants ranged from approximately 18,000 lux to 115,000 lux. Sessions with either no cloud cover (0%) or complete cloud cover (100%) tended to have low variability (smaller standard deviations) in illuminance across trials, while sessions with moderate cloud cover tended to have higher variability in horizontal illuminance across trials. As discussed below in the results section, neither illuminance level nor amount of cloud cover was significantly related to the probability of the participant seeing the detectable warning. Also, these two aspects of the lighting conditions were not significantly related to the probability of the participant giving a detectable warning a high conspicuity rating.

2.3 Visual Detection
For each sidewalk type, Table 4 shows the percentage of participants who were able to see each detectable warning at 2.44 m (8 ft) and at 7.92 m (26 ft). Although most of the detectable warnings tested in this study were seen by most participants at a distance of 2.44 m (8 ft), a few combinations of sidewalk type and detectable warning color were not seen by many participants. At greater distance, up to 7.92 m (26 ft), certain combinations of detectable warning color and sidewalk were more likely to be seen than others. Sidewalk color had an important influence on the number of participants who could see the single-color detectable warnings. Two-color (black-and-white) detectable warnings were seen by a high percentage of participants at all distances tested on all four sidewalk colors tested.

Table 4. Percentage of Participants (n = 50) Who Saw Each Detectable Warning at 2.4 m (8 ft) and 7.9 m (26 ft) for Each Sidewalk Type

Detectable Warning Colors	Brick		Asphalt		White		Brown
Detectable Warning Colors	Percent seen at 8 ft.	Percent seen at 26 ft.	Percent seen at 8 ft.	Percent seen at 26 ft.	Percent seen at 8 ft.	Percent seen at 26 ft.	Percent seen at 8 ft.	Percent seen at 26 ft.
White	96	86	98	88	66	28	98	86
Light Gray	84	50	98	78	94	76	86	58
White Concrete	94	80	98	88	36	10	100	82
Brown Concrete	68	32	98	64	98	84	36	8
Dark Gray	84	64	78	46	100	86	92	68
Federal Yellow	94	78	98	88	88	62	98	80
Pale Yellow	96	74	98	88	82	58	98	76
Bright Red	84	62	92	66	100	86	94	68
Orange-Red	76	56	92	68	98	84	86	66
Black	98	68	78	40	96	82	96	68
Black with White border	98	78	98	82	98	78	98	82
Black with White stripes	96	86	100	86	98	86	96	84
White with Black border	96	86	98	88	96	76	98	90

2.4 False Detections
On each of the white, brown, and asphalt sidewalks, each participant experienced two trials in which there was a flat blank panel on the sidewalk instead of a bumpy detectable warning. Out of 300 total blank trials, 11 (3.7 percent) resulted in “false alarms” or false detections where the participant reported seeing a detectable warning even though the target was a blank panel. Nearly half of the false detections were due to a single participant who reported seeing a detectable warning on 5 out of 6 of her blank trials. This participant’s results indicate that she responded appropriately to the actual detectable warnings despite the false detections on blank trials. If her data are set aside, the false alarm rate for the remaining group of 49 participants was 2 percent. The low false alarm rate is consistent with the instructions to participants to report that they could see a detectable warning when they were “confident” that there was a detectable warning on the sidewalk.

2.5 Visual Detection Distance
As described in the Method section, the maximum distance up to 7.92 m (26 ft) at which each participant could see each detectable warning was measured. These data were then analyzed at 1-foot intervals from 2.44 m (8 ft) to 7.92 m (26 ft) by counting the number of participants at each distance who were able to see the detectable warning. For this analysis, we assumed that participants who could see a particular detectable warning from a greater distance could also see it from closer distances. For example, if a participant first saw the detectable warning from a maximum distance of 4.3 m (14 feet), and saw the detectable warning when tested at the 2.44 m (8 ft) distance, then we assumed that the participant could also see the detectable warning at all intermediate distances between 2.44 m (8 ft) and 4.3 m (14 ft).

The results of this analysis are summarized in Figure 8 through Figure 20. In each of these figures, the percentage of participants (n = 50) who were able to see a particular detectable warning from distances between 2.44 m (8 ft) and 7.92 m (26 ft) is plotted at 1-foot intervals. For clarity of presentation, four different lines, corresponding to the four different sidewalk colors tested have been drawn to connect the data points. These lines show how the percentage of participants who were able to see the detectable warning changes with distance for the four different sidewalk colors tested. For any observed percentage value (P) plotted in Figure 8 through Figure 20, a 95-percent confidence interval for the true population percentage may be constructed by applying the formula:

For the reader’s convenience, confidence intervals for several values of P are given in Table 5, however, to assure clarity of presentation, no confidence bounds are shown in Figures 8 through Figure 20.

As expected, the percentage of participants who were able to see the detectable warning increases with distance from 7.92 m (26 ft) to 2.44 m (8 ft). Some of the detectable warning colors tested had much higher rates of visual detection at both 2.44 m (8 ft) and at 7.92 m (26 ft) than other colors. In cases where the sidewalk color is similar to the detectable warning color, the percentage of participants who were able to see the detectable warning is reduced. In fact, the least detectable combinations were the white “concrete” detectable warning on the white sidewalk and the brown “concrete” detectable warning on the brown sidewalk. In these cases, the detectable warning color and the sidewalk color were nearly identical.

Percentage	Lower bound	Upper bound
5%	0%	11%
10%	2%	18%
15%	5%	25%
20%	9%	31%
25%	13%	37%
30%	17%	43%
35%	22%	48%
40%	26%	54%
45%	31%	59%
50%	36%	64%
55%	41%	69%
60%	46%	74%
65%	52%	78%
70%	57%	83%
75%	63%	87%
80%	69%	91%
85%	75%	95%
90%	82%	98%
95%	89%	100%

Detection distances for the 10 single-color detectable warnings are shown in Figure 8 through Figure 17. In these 10 figures, the 4 plotted lines tend to spread apart, indicating that the percentage of participants who can see these detectable warnings depends on the sidewalk type. However, in Figure 18 through Figure 20, which show data for the black-and-white patterned detectable warnings, the four plotted lines tend to run close together, indicating that the percentage of participants who were able to see these black-and-white patterned detectable warnings does not depend strongly on sidewalk type. Overall, the black-and-white patterned detectable warnings tended to be seen by more participants than most of the single color detectable warnings.

2.5.1 Comparing Visual Detection Distances for Detectable Warnings
For practical purposes, someone trying to decide between two or more available detectable warning colors for a particular sidewalk application may want to know which of the colors could be seen by people with visual impairments from the greatest distance. As shown by the previous set of figures, visual detection depends on the color of the sidewalk as well as the color of the detectable warning. Thus, for a given sidewalk type, it is necessary both to know which of the detectable warning colors tested in this study could be seen at greater distances than other colors and to have a means to decide whether any observed differences in detection distance were statistically significant.

The data on maximum visual detection distance were constrained by the experimental procedure which permitted minimum and maximum viewing distances of 2.44 m (8 ft) and 7.92 m (26 ft). The data are strongly skewed, with many trials resulting in visual detection at a distance of 7.92 m (26 ft). Other trials resulted in detection at various distances between 2.44 m (8 ft) and 7.92 m (26 ft), and some trials did not result in detections. Although viewing distances closer than 2.44 m (8 ft) were not tested, the study team coded detection distance for trials with non-detections as zero feet. In order to compare the detection distances for the 13 detectable warnings, we performed a series of pairwise comparisons using a non-parametric statistic appropriate for repeated measures data. Using SAS statistical software, we computed the Wilcoxon Signed-Rank statistic for each of the 78 possible pairwise comparisons of the 13 detectable warnings on a given sidewalk. In order to control the probability of the statistical Type I error at .05 across the entire set of 78 comparisons, we used a criterion of α = .05 / 78 = .000641 for each pairwise comparison. This method for controlling Type I error is conservative. It controls the probability of claiming statistically significant differences between detectable warnings where no differences actually exist, however, this method may risk obscuring some interesting, and potentially real practical differences observed in this study. Therefore, in addition to reporting statistically significant differences in the conservative manner described above, we have also reported the direction of pairwise differences which would have been considered statistically significant if the two detectable warnings had been tested in isolation, rather than as part of a set of multiple comparisons. Thus, for this second less conservative statistical decision criterion we set a criterion of α = .05 for each pairwise comparison.

The results of the analyses are summarized in Table 6 through Table 9. Each of these tables corresponds with one of the four sidewalk types tested. The row and column headings refer to the detectable warning colors tested. The results of the pairwise comparison between detectable warnings listed in rows and those listed in columns are shown at the intersection of the appropriate row and column. The double “greater than” symbol (>>) indicates that the detectable warning color heading the row was seen from a significantly greater distance than the detectable warning color heading the column based on the conservative criterion where (p < .000641). The single “greater than” symbol (>) indicates that the detectable warning color heading the row was seen from a significantly greater distance than the detectable warning color heading the column based on the less conservative criterion where (p < .05). Similarly, the double “less than” symbol (<<) and single “less than” symbol (<) indicate that the detectable warning color heading the row was seen from significantly less distance than the detectable warning color heading the column based on the two criteria described above. The notation “n.s.” indicates that observed differences in detection distance were not statistically significant (p > = .05). For example, on the brick sidewalk the three black -and -white patterned detectable warnings were not significantly different from each other in terms of detection distance. Note that none of the significant differences designated by a single “greater than” or single “less than” symbol is statistically significant by the more conservative criterion.

The interaction between detectable warning color and sidewalk color is apparent from comparing the pattern of results across the four sidewalk types (Table 6 through Table 9). There are several cases where one detectable warning color may be seen from a greater distance than a second detectable warning color on a particular sidewalk type, but for a different sidewalk type, the second detectable warning color may be seen from a greater distance than the first. For example, the federal yellow detectable warning is seen from significantly greater distances than the dark gray detectable warning on all sidewalk types except for the white sidewalk, where the dark gray is seen from significantly greater distances than the federal yellow.

Table 6. Brick Sidewalk: Significant Differences in Visual Detection Distance for Detectable Warnings

	White	Light Gray	White Concrete	Brown Concrete	Dark Gray	Federal Yellow	Pale Yellow	Bright Red	Orange Red	Black	Black with White Border	Black/ White Stripes
Light Gray	<<
White Concrete	n.s.	>>
Brown Concrete	<<	<<	<<
Dark Gray	<<	>	<<	>>
Federal Yellow	<	>>	n.s.	>>	>>
Pale Yellow	<	>>	n.s.	>>	>>	n.s.
Bright Red	<<	n.s.	<<	>>	n.s.	<<	<<
Orange-Red	<<	n.s.	<<	>>	<	<<	<<	<
Black	<	>>	<	>>	>>	n.s.	n.s.	>>	>>
Black with White Border	n.s.	>>	n.s.	>>	>>	n.s.	>	>>	>>	>
Black/ White Stripes	n.s.	>>	n.s.	>>	>>	n.s.	>	>>	>>	>	n.s.
White with Black Border	n.s.	>>	n.s.	>>	>>	>	>	>>	>>	>	n.s.	n.s.

Table 7. Asphalt Sidewalk: Significant Differences in Visual Detection Distance for Detectable Warnings

	White	Light Gray	White Concrete	Brown Concrete	Dark Gray	Federal Yellow	Pale Yellow	Bright Red	Orange Red	Black	Black with White Border	Black/ White Stripes
Light Gray	<
White Concrete	n.s.	>
Brown Concrete	<<	<	<<
Dark Gray	<<	<<	<<	<<
Federal Yellow	n.s.	>	n.s.	>>	>>
Pale Yellow	n.s.	n.s.	n.s.	>>	>>	n.s.
Bright Red	<<	<	<<	n.s.	>>	<<	<<
Orange-Red	<<	<	<<	n.s.	>>	<<	<<	n.s.
Black	<<	<<	<<	<<	n.s.	<<	<<	<<	<<
Black with White Border	<	n.s.	n.s.	>>	>>	n.s.	n.s.	>	>>	>>
Black/ White Stripes	n.s.	>	n.s.	>>	>>	n.s.	n.s.	>>	>>	>>	n.s.
White with Black Border	n.s.	>	n.s.	>>	>>	n.s.	n.s.	>>	>>	>>	n.s.	n.s.

Table 8. White Concrete Sidewalk: Significant Differences in Visual Detection Distance for Detectable Warnings

	White	Light Gray	White Concrete	Brown Concrete	Dark Gray	Federal Yellow	Pale Yellow	Bright Red	Orange Red	Black	Black with White Border	Black/ White Stripes
Light Gray	>>
White Concrete	<<	<<
Brown Concrete	>>	>	>>
Dark Gray	>>	>	>>	n.s.
Federal Yellow	>>	<	>>	<<	<<
Pale Yellow	>>	<<	>>	<<	<<	<
Bright Red	>>	>>	>>	n.s.	n.s.	>>	>>
Orange-Red	>>	>	>>	n.s.	n.s.	>>	>>	n.s.
Black	>>	>	>>	n.s.	n.s.	>>	>>	n.s.	n.s.
Black with White Border	>>	n.s.	>>	n.s.	<	>	>>	<	n.s.	n.s.
Black/ White Stripes	>>	>	>>	n.s.	n.s.	>>	>>	n.s.	n.s.	n.s.	n.s.
White with Black Border	>>	n.s.	>>	<	<	>	>	<	<	<	n.s.	<

Table 9. Brown Concrete Sidewalk: Significant Differences in Visual Detection Distance for Detectable Warnings

	White	Light Gray	White Concrete	Brown Concrete	Dark Gray	Federal Yellow	Pale Yellow	Bright Red	Orange Red	Black	Black with White Border	Black/ White Stripes
Light Gray	<<
White Concrete	n.s.	>>
Brown Concrete	<<	<<	<<
Dark Gray	<<	>	<<	>>
Federal Yellow	<	>>	n.s.	>>	>>
Pale Yellow	<	>>	n.s.	>>	>>	n.s.
Bright Red	<<	>	<<	>>	n.s.	<	<
Orange-Red	<<	n.s.	<<	>>	<	<<	<<	<
Black	<<	>>	<	>>	>	<	<	n.s.	>>
Black with White Border	n.s.	>>	n.s.	>>	>>	n.s.	n.s.	>	>>	>
Black/ White Stripes	n.s.	>>	n.s.	>>	>>	n.s.	n.s.	>	>>	>	n.s.
White with Black Border	n.s.	>>	n.s.	>>	>>	>	>	>>	>>	>>	n.s.	n.s.

Row vs. Column differences are indicated by double or single greater than (>) or less than (<) symbols (n.s. = "not significant"). Statistically significant differences are based on the Wilcoxon Signed-Rank test (two-tailed) performed for each pair of detectable warnings. Double and single symbols indicate statistically significant differences where p < .000641 or p < .05 respectively. For each comparison shown, the row heading should be read before the column heading. For example, participants’ visual detection distances for the light gray detectable warning were significantly less than their visual detection distances for the white detectable warning.

2.6 Conspicuity Ratings
Participants rated the conspicuity of each detectable warning that they could see on a numerical scale that ranged from 1 (very unlikely to attract my attention on this type of sidewalk) to 5 (very likely to attract my attention on this type of sidewalk). A rating of zero (or “X” in the figures below) was assigned by the experimenter to those detectable warnings that were not detected from the viewing distance

ACCESS BOARD RESEARCH

Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments

Final Report PDF version Technical Brief

ACCESS BOARD RESEARCH

Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments Final Report PDF version Technical Brief

Visual Detection of Detectable Warning Materials by Pedestrians with Visual Impairments

Final Report PDF version Technical Brief