Why Fixed Slip Devices Can Not Measure The Speed Gradient Due To The Pavement

This paper covers why measurements at various speeds with continuous friction measurement equipment (CFME) are not able to measure the speed - friction gradient of the pavement as determined by the macro-textural features of the surface. Most CFMEs measure friction in the slip ratio range of 10% to 18%. In this range it is shown that the friction versus slip speed of these devices are mainly determined by the coupled properties of the surface micro-texture and relevant tire properties and to a minimal extent only by pavement macro-texture properties. The determinacy of low slip ratio friction measurements on the macro-textural features of the pavement surface are shown to be so low that they are inadequate to capture macro-texture caused speed degradation of friction. It is also shown in the paper that close to 100% slip ratio measurements at a relatively wide speed scale or direct macro texture measurements are needed to adequately determine the speed gradient if friction caused by the pavement macro-texture. The data and analysis in this paper provides strong and compelling evidence that the European Friction Index or EFI developed by the HERMES project sponsored by FEHRL is strongly affected by the lack of consideration of this phenomenon. It is recommended that the results of the HERMES project are reviewed taking into account the findings of the present paper. Based on the results of the presented paper it is suggested that CFME’s only measure friction at a fixed speed with a standard tire and use a macro-texture measuring device to obtain the pavement speed gradient.