This paper presents an investigation directed at better understanding theprinciples underlying the concept of the International Friction Index (IFI), and specifically the role played by the Speed Constant (Sp) parameter in the IFI computations. The parameter Sp dictates the speed variation of the wet friction measurements taken on a given pavement surface. Although there is ample evidence in the literature associating the Sp parameter to only the macrotexture characteristics of pavement surfaces, stronger correlations were obtained when the Sp parameters were related to the pavement macrotexture in device specific fashion. Hence the results of the current investigation suggest the revision of the procedure for computation of theSp parameter to incorporate vehicle specific properties in it as well. The necessity for revision is further supported by the fact that the speed gradient of friction on any interface must depend on the characteristics ofboth contact surfaces, which in this case are the pavement and the evaluation devices. The incorporation of vehicle characteristics in the Sp parameter computations would help address a well known deficiency of the 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. Due to the above 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 Sp parameter reduces significantly the slip speed dependency of the device calibration parameters A and B.
Samenvatting