Specifications

• Size
• Detectable warnings shall be 24 inches [610 mm] in the direction of travel and extend the full width of the curb ramp or flush surface.
• Research has confirmed that for people 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. 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
• Location
• Detectable warning shall be located so that the edge nearest the curb line or other potential hazard is 6 to 8 inches [150 to 205 mm] from the curb line or other potential hazard.
• 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.

Dome Size and Spacing

• The original ADA design standard for truncated domes is found in ADAAG (4.29.2). After the research was conducted, a new design recommendation was made for the dimension and placement of the domes on curb ramps. Both FHWA and the U.S. Access Board are encouraging the use of the new design over the original. Information on the recommended design and other useful information are included.
• Truncated domes shall have a base diameter of 0.9 inches [23 mm] minimum to 1.4 inches [36 mm] maximum, a top diameter of 50% or 0.45inch [10 mm] to .9 inches [23 mm] at the top, a height of 0.2 inch [5 mm] and a center-to-center spacing of 1.6 inches [41 mm] to 2.4 inches [61 mm] measured along one side of a square arrangement.
• 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 bottom diameter measures 0.9-1.4 inches, 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 to 0.45 of an inch is the suggested, as based on current research.

Dome Alignment

• Domes shall be aligned on a square grid in the predominant direction of travel to permit wheels to roll between domes. This specification ensures a greater degree of safety and negotiability for persons with mobility impairments. It requires square alignment, to give people using wheeled mobility aids the greatest chance of being able to avoid the truncated domes.

Visual Contrast

• There shall be a minimum of 70% contrast in light reflectance between the detectable warning and an adjoining surface. The material used to provide visual contrast shall be an integral part of the detectable warning surface. Refer to your local committee on visual contrast, as some suggest that a lesser level of contrast could be as effective and more economical to provide than a minimum 70%.