Avoid stumbling blocks to access. (National Therapeutic Recreation Society).

Walking near the edge of a pool or ledge makes many people nervous. People with disabilities have an ever greater need to take care. That's why many facilities are including detectable warning surfaces in their designs.

According to federal guidelines, a detectable warning is "a standardized surface feature built in or applied to walking surfaces or other elements to warn of hazards on a circulation path." It's a unique and standardized feature, intended to function much like a stop sign and to alert pedestrians who are visually impaired to the presence of a hazard in the line of travel.

As curb ramps have become common in response to the requirements of the Rehabilitation Act and the Americans with Disabilities Act, an unintended consequence has been that blind pedestrians have found it more difficult to locate the boundary between the street and sidewalk. The only surface that has repeatedly been demonstrated to be detectable to most blind pedestrians, either under foot or by the use of a long cane, is the truncated-dome detectable warning surface, which has been required on transit platform edges since 1991 and at curb ramps since July 2001.

According to the draft guidelines for Accessible Public Rights-of-Way, specifications to consider include:

* Detectable warning surfaces should have truncated domes aligned in a square grid pattern.

* The truncated domes should have a base diameter of 0.9 inches (23 mm) to 1.4 inches (36mm), a top diameter of 50 to 65 percent of the base, and a height of 0.2 inches (5 mm).

* The center-to-center spacing between the domes should be 1.6 inches (41 mm) minimum to 2.4 inches (61 mm) and a base-to-base spacing of 0.65 inches (16mm) minimum, measured between the most adjacent domes on square grid.

* The warning surfaces should noticeably contrast with adjoining surfaces, either light-on-dark or dark-on-light.

There are a number of truncated-dome products available for installation on curb ramps in various climates and conditions. A report on detectable warnings has been developed by the Access Board and is available by calling 800-872-2253 and asking for "Detectable Warnings: Synthesis of U.S. and International Practice." The publication is also available online at www.access-board.gov.