Effect of Pavement Roughness and Road Illumination on the Free Flow Speed in Highways

This study is aimed at evaluating the effects of pavement roughness and darkness on the free-flow speed in the freeways. In general, most of valid codes such as Highway Capacity Manual (HCM), do not consider the effect of pavement roughness and darkness on the free-flow speed, capacity and service level of roads. In order to determine the pavement roughness condition, international roughness index (IRI) measured by Road Surface Profiler (RSP), manufactured by Dynatest Company, has been employed. Moreover, the relevant data on the pavement roughness and free-flow speed of vehicles have been recorded in the light and dark points at 16 sections of a freeway with the length of 62 km. Speed Gun device, traffipatrol XR model, has been utilized to measure free flow speed. This device is capable to measure speed within more than one kilometer at days and nights. In order to prevent the effects of speed controlling on the behavior of drivers, the device was located in places where were not in the horizon of drivers. The results of the current study indicated that the free flow speed can be explained by the linear-inverse model based on road roughness at significant level of 0.01. Besides, the sensitivity of free flow speed to the roughness in the darkness is lower than that in the lighting. Finally, in this study typical factors to modify the roughness condition and darkness in the equations of free flow speed in the HCM method are proposed.