Truncated domes are the standard design requirement for detectable warnings for determining the boundary between the sidewalk and street by people with visual disabilities.
The Department of Justice (DOJ) is the lead agency that oversees the Americans with Disabilities Act (ADA). Detectable warnings were required in 1991 by the Americans with Disabilities Act Accessible Guideline (ADAAG) for hazardous vehicular ways, transit platform edges, and curb ramps
Truncated domes have a unique design that can be detected underfoot and with a cane, and other surfaces are not considered ADA equivalent and therefore do not comply with the ADA requirements.
Per federal law, there are several guidelines of which a truncated dome can be manufactured. Some of those guidelines are listed below:
Detectable warnings shall consist of a surface of truncated domes aligned in a square or radial grid pattern and shall comply with R304.
R304.1.1 Dome Size.
Truncated domes in a detectable warning surface shall have a base diameter of 23 mm (0.9 in) minimum to 36 mm (1.4 in) maximum, a top diameter of 50 percent of the base diameter minimum to 65 percent of the base diameter maximum, and a height of 5 mm (0.2 in).
Where domes are arrayed radially, they may differ in diameter within the ranges specified.
Truncated domes in a detectable warning surface shall have a center-to-center spacing of 41 mm (1.6 in) minimum and 61 mm (2.4 in) maximum, and a base-to-base spacing of 17 mm (0.65 in) minimum, measured between the most adjacent domes.
Detectable warning surfaces shall contrast visually with adjacent gutter, street or highway, or walkway surfaces, either light-on-dark or dark-on-light.
There are several other considerations defined by ADA regulations.