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

1.3 Previous Research

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:

• To provide a visual cue to identify the presence of a hazard (intersection between walkway and street).

• To function like a stop sign and direct appropriate behavior by warning pedestrians to stop and determine the nature of the hazard before proceeding.

• To provide a visual cue for locating the curb ramp (and crosswalk). The detectable warning is usually a useful point of departure for crossing the street.

• To provide a visual cue for orienting away from the departure curb, by aligning perpendicular to the detectable warning edge. This may be especially useful where the orientation of the curb-roadway boundary is difficult to detect, however, this cue is somewhat unreliable. Depending on the geometry of the street and crosswalk the detectable warning may not be installed at an angle perpendicular to the crosswalk.

• To provide a visual cue for the destination on the opposite side of the street. The detectable warning may serve as a visual cue to guide pedestrians to the destination curb ramp, helping them to maintain a proper travel path while crossing.

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.