Injuries and deaths from falls due to slipping on walking surfaces have been recognized as a major accident problem in many countries. The safety of walking depends primarily on the surface slip resistance, which is defined as the ratio of (a) the minimum shear force necessary to initiate slipping of a body over the surface to (b) the body's normal force. Several kinds of devices are currently in use to measure slip resistance. In this paper three slip resistance measuring devices are evaluated: the National Bureau of Standards-Brungraber Tester, the Horizontal Pull Slipmeter, and the Pennsylvania Transportation Institute Drag Sled Tester. The testing program consisted of two phases. In the first phase each tester was used to measure the slip resistance of selected surfaces representing a wide range of frictional characteristics of typical indoor and outdoor surfaces. All tested materials were in actual use on walking surfaces. In the mechanical tests the testers were evaluated with respect to their applicability, precision, repeatability, and sensitivity to the operator's measuring technique. The correlation was determined between the results produced by the testers on the same surfaces.
In the second phase of the testing program a series of experiments was conducted with human subjects walking over surfaces having relatively low slip resistance. The objective was to determine whether people will slip more often on surfaces which are measured as having lower slip resistance. In general, the agreement between the results of slip resistance measurements and the results of the biomechanical tests was high. It was observed, however, that the process of slipping involves a considerable randomness.