X02.5.6.2 Detectable Warnings. Curb ramps at medians and refuge islands, and locations where medians and refuge islands are cut through level with the street at crosswalks, shall have detectable warnings complying with Section X02.5.7.

X02.5.7 Detectable warnings

X02.5.7.1 General. Where required, detectable warnings shall comply with Section X02.5.7.

X02.5.7.2 Application. Detectable warnings shall be provided only at the following locations:

(A) Where a sidewalk crosses a vehicular way, excluding unsignalized driveway crossings.

Figure X02.5 G Detectable Warning at Curb Ramp

Illustrates 24" deep detectable warning located near the street edge of the curb ramp.

Figure X02.5 H Transition Ramp with Detectable Warning

Shows detectable warning at a transition ramp.

Figure X02.5 I Shared Curb Ramp with Detectable Warning

Shows detectable warning at a shared curb ramp.

Figure X02.5 J Detectable Warning at Blended Curb

Shows detectable warning at blended curb.

Figure X02.5 K Detectable Warnings at Multi-Use Path

Plan view of a multi-use path and road intersection. Detectable warnings are indicated at the intersection.

(B) Where a rail system crosses pedestrian facilities that are not shared with vehicular ways.

Figure X02.5 L Detectable Warnings at Railroad Crossing

Plan view of detectable warnings at a railroad crossing.

(C) At reflecting pools within the public right-of-way, which have no curb or rim protruding above the walking surface.

(D) At islands and medians that are cut through level with the roadway.

Figure X02.5 M Refuge Island with Detectable Warnings

Plan view of pedestrian passage that cuts through a refuge island at the same level as the street. Detectable warnings are shown at each end of the cuts.

Advisory: Where islands or medians are less than 4 feet wide, the detectable warning should extend across the full length of the cut through the island or median.

(E) Where required by proposed ADAAG Chapter 10.

Discussion: The detectable warning is a unique and standardized surface intended to function much like a stop sign to alert pedestrians who are blind or visually impaired to the presence of hazards in the line of travel. The truncated dome surface should not be used for wayfinding or directional information. The removal of curbs, which provided a clearly defined indication of the location of the edge of the street, has caused difficulty for individuals who are blind or visually impaired. The locations above were identified by the committee as being appropriate for the installation of detectable warnings. Detectable warnings are not required at unsignalized driveways based on comments to the committee that installation at driveways would make it harder to truly identify the streets.

X02.5.7.3 Specifications.

(A) Size. Detectable warnings shall be 24 inches (610mm) in the direction of travel and extend the full width of the curb ramp or flush surface.

Discussion: Research has confirmed that for persons who are visually impaired, there is a high level of risk of inadvertent street entry associated with the presence of curb ramps, particularly those having slopes of 1:12 or less (Bentzen, B. & Barlow, J., 1995; Hauger, S., Rigby, J., Safewright, M. and McAuley, W., 1996). It has been demonstrated that detectable warnings complying with existing ADAAG Section 4.29.2 are highly detectable by persons with visual impairments, and can provide an effective stop signal for persons who are blind or visually impaired which can be used to determine the end of the sidewalk and the beginning of the vehicular way. Research has also demonstrated that 24 inches of detectable warning material is sufficient to enable persons who are blind or visually impaired to stop on 90 percent of approaches (Peck, A. & Bentzen, B., 1987).

Research has now been conducted which addresses concerns about safety of detectable warnings for individuals with mobility impairments, indicating that detectable warnings on slopes have minimal impact on the safety and ease of travel for persons having physical disabilities (Bentzen, B., Nolin, T., Easton, R., Desmaris, P., and Mitchell, P., 1994; Hauger, et al, 1996). On the basis of this research, the committee voted to recommend the installation of detectable warnings at sidewalk/street transitions.

A few committee members did not fully support this recommendation, feeling there might be a significant adverse impact on safety and ease of travel for wheelchair users. The committee discussed threshold ramp grade requirements where only the gentlest ramps (1:15 and flatter) would have detectable warnings. Nonetheless, because such a requirement would tend to confuse both designers and builders and would give inconsistent information to individuals who are visually impaired, the committee voted to require detectable warnings on all sidewalk/street transitions regardless of slope.

(B) Location. The detectable warning shall be located so that the edge nearest the curb line or other potential hazard is 6 to 8 inches (150 to 205mm) from the curb line or other potential hazard, such as a reflecting pool edge or the dynamic envelope of rail operations.

Discussion: Placement of the detectable warnings a maximum of 6 to 8 inches back from the curb line gives some latitude in placement of the detectable warning. Where curbing is embedded at the sidewalk/street junction, this will not need to be replaced. In addition, allowing 6 to 8 inches of ramp (or curb) surface beyond the detectable warning will give pedestrians who are blind an additional stopping distance before they step into the street. It will also enable some persons having mobility impairments to make a smoother transition between the street and the curb ramp.

(C) Dome size and spacing. Truncated domes shall have a diameter of 0.9 inch (23 mm) at the bottom, a diameter of 0.4 inch (10 mm) at the top, a height of 0.2 inch (5 mm) and a center-to-center spacing of 2.35 inches (60 mm) measured along one side of a square arrangement.

Figure X02.5 N Dome Section

Section of dome from a detectable warning. Drawing shows height, top and bottom dimensions.

Figure X02.5 O Dome Spacing

Plan and section views of detectable warning domes and their relative spacing on the x and y axis.

Discussion: The size and spacing of the domes affect detectability by pedestrians who are blind. This specification is much more detailed than that in the current ADAAG, and offers much less latitude in dimensions and spacing. It ensures that the dome spacing is the maximum currently known to be consistent with high detectability. The diameter measurement in the present ADAAG is ambiguous if the user of these guidelines is not told whether the diameter is to be measured at the bottom or the top of the truncated domes. As currently implemented by most US manufacturers, it is the bottom diameter that measures 0.9 inch, and the top diameter varies widely. The diameter of the dome where it touches the sole of the shoe affects detectability, and the top diameter of 0.4 inch, in the suggested language, is based on current research (see below).

A few members of the committee felt that there needed to be more flexibility in the size and shape of the domes. Some suggestions were that the domes be a semi-spherical shape using a 1-inch base, or a "butte" design with a larger top diameter (0.6 inch). Wider spacing, up to 3 inches, between domes was also suggested. It was felt that the wider gaps or lanes between the domes would better accept the wheel path of most wheelchairs so that users would not need to "bump" over the domes. However, there was no evidence that either of these alternatives would be better or worse than the proposed standard in terms of ease of traversal by wheelchair users and detectability for individuals who are blind or visually impaired. The proposed standard is supported by research on spacing and detectability completed in Japan in 1998. The committee voted to recommend the parallel alignment of domes as well as the two-foot depth of the detectable warning, in consideration of minimizing bumpiness for wheelchair users.

(D) Dome alignment. Domes shall be aligned on a square grid in the predominant direction of travel to permit wheels to roll between domes.

Figure X02.5 P Dome Alignment

Plan view of a detectable warning surface showing domes aligned in rows, not skewed diagonally.

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