What values are recommended for ground and floor surfaces along an accessible route?
The surfaces of the accessible route on a site or within a building or facility must be designed to provide slip-resistant locomotion for both level and inclined travel by persons with disabilities. Research findings suggest that such surfaces should have a slip resistance somewhat higher than might be provided for individuals without disabilities.
In the study sponsored by the Access Board, laboratory measurements from a Kistler force plate and computer analysis of the gaits of persons with mobility impairments (including crutch users and above- or below- knee amputees using artificial limbs) and persons without disabilities graphed the dynamic coefficients of friction necessary for safe ambulation. The m-shaped curves that resulted gave a range of values from touch-down to take-off (control group: 0.2-0.3; persons with disabilities 0.7-1.0). Wheelchair users were tested through a full cycle of push and recovery (0.5-0.7).
Correlating these values with a single static coefficient of friction (the relationship is complex and non-linear) is inexact and involves some approximation in order to facilitate simplified field testing procedures. In the Access Board research, the static coefficients of friction for a variety of common indoor and outdoor surfacing materials were measured in place using the NBS-Brungraber Tester with a silastic sensor material. Although this machine operates on a principle similar to that of the James machine, the use of a non-standard silastic sensor (instead of the leather required by the protocol for the UL standard) results in significantly higher values for the coefficient of friction of the surfaces being measured. As no correlation was made to any other standards or methodologies in the research, the values for coefficient of friction cannot be compared.
Researchers' recommendations for a static coefficient of friction for surfaces along an accessible route, when measured by the NBS-Brungraber machine using a silastic sensor shoe, were approximately 0.6 for a level surface and 0.8 for ramps. These values are included in the advisory material in the Appendix to ADAAG, but are not in any way mandatory.
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