The ADA Accessibility Guidelines for Buildings and Facilities (ADAAG) defines detectable warnings as “a standardized surface feature built in or applied to walking surfaces or other elements to warn visually impaired people of hazards on a circulation path.”37 Detectable warnings have a raised grid of small flat-top domes (truncated domes) which is the standard texture element required for use on curb ramps or at other flush transitions between sidewalks and streets where there may not be sufficient cues for a visually impaired pedestrian to be able to detect the transition. In the United States, detectable warnings have been used to mark mobility hazards such as the boundary between sidewalk and street, the edge of a train platform, or the edge of a reflecting pool. The history of federal rule-making concerning detectable warnings has been summarized by Chandler (2004). Key points include:

  • The Americans with Disabilities Act, passed by Congress in 1990, requires that design criteria be established for building and altering commercial and public facilities. Since then, the U.S. Department of Transportation (USDOT) and U.S. Department of Justice (USDOJ) have had responsibility to develop regulations to implement the goals of this law. The U.S. Access Board, which is an independent federal agency, develops guidelines for new or altered facilities and vehicles to ensure accessibility.
  • The U.S. Access Board developed and maintains the ADA Accessibility Guidelines for Buildings and Facilities (ADAAG). These guidelines are referenced by regulations from USDOT and USDOJ in 1991. The ADAAG requires detectable warnings on the full surface of curb ramps.
  • In 1994 the USDOJ, USDOT, and U.S. Access Board temporarily suspended the requirement for detectable warnings. This suspension expired in 2001.
  • In 1999, the U.S. Access Board formed the Public Rights-of-Way Access Advisory Committee (PROWAAC) to make recommendations on the accessibility provisions in the ADAAG for sidewalks and streets.
  • In 2001, PROWAAC delivered its recommendations, which include changes to the specifications for detectable warnings in ADAAG.
  • In 2002, the U.S. Access Board published draft guidelines for public rights-of-way that include changes to detectable warning requirements provided by PROWAAC.
  • Currently the draft guidelines are being considered in the rule making process by USDOT and USDOJ to become enforceable standards. Until then, the 1991 standards remain legal requirements, however, USDOT and the U.S. Access Board have encouraged states to use the Draft Guidelines for Accessible Public Rights-of-Way (2002) for detectable warnings as an equivalent facilitation until the rulemaking process is completed.
  • In 2004, the U.S. Access Board published a new set of guidelines for accessible design in buildings and facilities. This set of guidelines requires detectable warnings at transit platforms. The requirement to use detectable warnings at curb ramps and blended transitions will be addressed in separate public rights-of-way regulations.
  • On November 23, 2005, the U.S. Access Board published a notice of availability of draft guidelines on accessibility in the public right-of-way (Federal Register, Vol. 70, No. 255). The Draft Public Rights-of-Way Accessibility Guidelines announced is the second draft available to the public. It incorporates changes based on over 1400 comments received from the public on the 2002 draft.

Those specifications for detectable warnings from ADAAG which are most important to the current project on the visibility of detectable warnings include:

“A curb ramp shall have a detectable warning[....] The detectable warning shall extend the full width and depth of the curb ramp.” (Note below that in the current Draft Guidelines for Accessible Public Rights-of-Way, the depth requirement has been reduced to 24 inches.)

The appendix to the ADAAG (A4.29) recommends that detectable warnings contrast visually with adjoining surfaces:

The material used to provide contrast should contrast by at least 70 percent. Contrast in percent is determined by:

Contrast = [(B1-B2)/B1] x 100

B1 is the light reflectance value of the lighter area
B2 is the light reflectance value of the darker area
Note that in any application both white and black are never absolute: thus, B1 never equals 100 and B2 is always greater than 0.

Specifications from the current set of Draft Guidelines for Accessible Public Rights-of-Way (November, 2005) include the following:

Dome size

Height of domes: 5 mm (0.2 inch)
Base diameter: 23 mm to 36 mm (0.9 to 1.4 inches)
Top diameter: 50% to 65% of the base diameter

Dome spacing

Pattern: Square grid pattern
Center-to-center: 41 mm to 61 mm (1.6 to 2.4 inches)
Base-to-base: 17 mm (0.65 inch) minimum, between closest domes in grid

Overall dimensions: The detectable warning surface containing the domes must be 61 cm (24 inches) in the direction of travel and as wide as the curb ramp (exclusive of flares).

Visual properties: Detectable warnings shall contrast visually with adjacent gutter, street or highway, or walkway surface, either light-on-dark, or dark-on-light. An additional advisory note says that: “Contrast may be provided on the full ramp surface but should not extend to the flared sides. Many pedestrians use the visual contrast at the toe of the ramp to locate the curb ramp opening from the other side of the street.”

The ADAAG requirement for 70 percent contrast is not contained in the current draft guidelines. Concern has been expressed that the requirement may be difficult to achieve with available paving materials, and there was no standard measurement procedure specified for determining if a detectable warning surface is in compliance with the 70 percent contrast requirement. The current draft guideline for visual contrast of detectable warning surfaces “to contrast visually with adjoining surfaces, either light-on-dark, or dark-on-light” is less specific than those in ADAAG, and not quantifiable.