Evaluation of the Speed Constant and Its Effect on the Calibration of Friction-Measuring Devices

Abstract

The speed constant (S_p) parameter dictates the speed variation of the wet friction measurements taken on a given pavement surface. Although ample evidence exists in the literature associating the S_p parameter only to the macrotexture characteristics of pavement surfaces, stronger correlations were obtained when the S_p parameters were related to the pavement macrotexture in device-specific fashion. The results of the current investigation suggest the revision of the procedure for computation of the S_p parameter to incorporate device-specific properties in it as well. The necessity for revision is supported further by the fact that the speed gradient of friction on any interface must depend on the characteristics of both contact surfaces, which, in this case, are the pavement and the evaluation devices. The incorporation of vehicle characteristics in the S_p parameter computations might address a well-known deficiency of the international friction index (IFI), which is the inconsistent FR60 (predicted friction at 60 km/h) obtained from the friction values measured at two different slip speeds on the same surface. Because of this inconsistency, the device calibration parameters A and B used in the computation of the IFI parameters are sensitive to the slip speed of the measuring device. This study also showed that the modification of the S_p parameter reduces significantly the slip speed dependency of the device calibration parameters A and B.

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