عنوان مقاله [English]
In general, the studies of free-flow speed are conducted under conditions of even pavement and day-light. However, 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. This study is aimed at deriving a relation between free-flow speed in the highways and pavement roughness and darkness. In this research, international roughness index (IRI) has been employed to determine the conditions of pavement roughness. Moreover, the relevant data to the pavement roughness and free-flow speed of vehicles have been recorded at 16 sections of a freeway with the length of 62 km. The current study proposes the modification factors for roughness using HCM method to predict free-flow speed in the highways. The obtained results revealed that free-flow speed has an inverse and linear relation with road roughness and its sensitivity to the roughness is lower at nights. Furthermore, the sensitivity of free-flow speed to the darkness is higher at small roughness.