Determination of Pavement Macrotexture Limit for Use in International Friction Index Model

Macrotexture is a fundamental parameter used to characterize the frictional properties of a pavement surface. In models such as the international friction index (IFI), macrotexture is used to quantify the speed sensitivity of wet friction measurements on a given pavement surface. IFI is primarily a statistical model, in which the pavement macrotexture is correlated to the speed constant, Sp, or the slope of the linear regression relationship between ln(friction) and the slip speed. An evaluation is presented of the statistical significance of pavement macrotexture on the IFI model and the applicability of IFI concepts to the range of macrotexture typical of highway and runway pavements. The results of the investigation showed that the proposed concept in the IFI model was not applicable for high macrotexture levels in particular because friction measured on such pavements was known to remain constant with changing slip speeds. At high levels of macrotexture, variations of friction measurements with slip speed could be explained by the error term used in linear regression. Moreover, the speed constant used in the IFI model was shown to be extremely sensitive at high levels of macrotexture from a theoretical perspective as well. The findings limit the applicability of the IFI model at relatively high levels of macrotexture. An appropriate threshold macrotexture level was determined for reliable application of the IFI model.