Practical Approach to Measuring and Reporting Friction and Macrotexture at Variable Test Speeds

Because of safety concerns associated with friction testing on both high- and low-speed facilities, testing at variable speeds has been investigated by the Florida Department of Transportation (DOT) and others. Previous research has demonstrated that reasonable correlations may be developed between friction test data obtained at the standard test speed of 40 mph (64.4 km/h) and other speeds. As part of an effort to harmonize its fleet of full-scale, locked-wheel friction testing equipment with portable reference test equipment (ASTM E 1960), the Florida DOT also evaluated the skid number speed gradient (G,) for use In transforming friction and macrotexture measurements obtained at variable highway speeds. This paper documents the results of this evaluation. On the basis of the results of this study, a practical method of measuring pavement friction and macrotexture at variable highway speeds with the Florida DOT locked-wheel test unit and the ribbed tire is presented. Since the results of the ribbed tire test are known to be significantly influenced by pavement microtexture and since the mean profile depth (MPD), as measured with the high-speed laser, is a direct measure of macrotexture, the friction number (FN)(40R) and the complementary MPD data together may be readily employed to characterize the frictional properties of the pavement. This approach builds on Florida's historical database of locked-wheel, ribbed-tire test data, while adding a direct measure of macrotexture and the flexibility of testing at variable speeds. Practical implementation recommendations and examples are provided.