Application of direct and indirect methods for measuring free-flow speed on roadways

Abstract

Free flow speed (FFS) is the drivers’ desired speed on roadways at low traffic volume and with the absence of traffic control devices. The determination of FFS is a fundamental step in the analysis of two-lane highways. FFS can either be determined using field measurements or estimated using existing empirical models. Malaysian Highway Capacity Manual (MHCM) provides a model for estimating FFS based on base-free-flow-speed (BFFS), roadway’s geometric features and proportion of motorcycles in the traffic stream. This model was compared against observed FFS obtained using Moving Car Observer (MCO) method which was in accordance with the suggested two-way flow rate below 200 veh/h, by the Highway Capacity Manual (HCM). Mean speeds were observed at higher flow rates and adjusted accordingly using a model provided by the HCM. Moving car observer method was used for collecting the relevant data related to travel time, speed, flow rate and traffic composition, by using a video recording instrumented test vehicle. The roadway geometric features were measured manually. Field measurements were carried out on ten directional segments of rural two-lane highways in Johor, Malaysia. Field data obtained were analyzed to estimate FFS on each segment. Statistical analyses indicate that there is a statistically significant difference between the FFS observed using MCO method and the estimated FFS using MHCM method. Such a finding suggests that there is a need to review the validity of the model provided in the MHCM for accuracy of the analyses