Detectable warnings are a distinctive surface pattern of domes detectable by cane or underfoot that alert people with vision impairments of their approach to street crossings and hazardous drop-offs. They are used to indicate the boundary between pedestrian and vehicular routes where there is a blended transition rather than a curbed connection. Detectable warnings also indicate unprotected drop-offs along the edges of boarding platforms at transit stations and stops.
Resource: United States Access Board
There are two different types of dome configuration:
The in-line spacing of the domes allows the visually impaired to detect the direction of the domes within the detectable warning tile by foot as they navigate sidewalk curb ramps, blended transitions, island medians or other locations. The inline or parallel alignment of domes also allows for wheeled devices to make a smoother transition between the street and sidewalk accessing via the curb ramp.
ADAAG // Article 705 relates the federal regulations with regards to Detectable Warnings.
705.1 General. Detectable warnings shall consist of a surface of truncated domes and shall comply with 705.
705.1.1 Dome Size. Truncated domes in a detectable warning surface shall have a base diameter of 0.9 inch (23 mm) minimum and 1.4 inches (36 mm) maximum, a top diameter of 50 percent of the base diameter minimum to 65 percent of the base diameter maximum, and a height of 0.2 inch (5.1 mm).
705.1.2 Dome Spacing. Truncated domes in a detectable warning surface shall have a center-to-center spacing of 1.6 inches (41 mm) minimum and 2.4 inches (61 mm) maximum, and a base-to-base spacing of 0.65 inch (17 mm) minimum, measured between the most adjacent domes on a square grid.
705.1.3 Contrast. Detectable warning surfaces shall contrast visually with adjacent walking surfaces either light-on-dark, or dark-on-light.
Figure 705.1 Size and Spacing of Truncated Domes, Resource: ADA.gov
Dome Spacing: as mentioned in 705.1.2, a range of different dome spacings is permitted. The most common spacing between the domes are either 1.67” (narrow) or 2.35” (wide).
StrongGo® Industries has a complete inventory of TekWay® ADA Domes with both 1.67” and 2.35” dome spacing to meet most needs of any specific project.
TekWay® ADA Domes with 2.35” Dome spacing