Comparison of free flow speed estimation models

Abstract

Free flow speed (FFS) is the drivers’ desired speed on roadways at low traffic volume and absence of traffic control devices whose determination is a fundamental step in the analysis of two-lane highways. FFS can either be estimated using either analytical model or based on field measurement. Regarding the former approach; the Malaysian Highway Capacity Manual (MHCM) established a model for estimating FFS based on base-free-flow-speed (BFFS), roadway’s geometric features and fraction of motorcycles in the traffic stream. On the other hand, the Highway Capacity Manual (HCM) suggested an approach for field measurement of FFS; preferably at a two-way flow rate not exceeding 200 veh/h. For many highways, observing a two-way flow rate of 200 veh/h or less is seldom met or impossible. In such situations, mean speed could be observed at higher flow rates and adjusted accordingly using a model provided by the HCM. This study describes the application of the two approaches for measuring FFS. Moving car observer (MCO) method was used for collecting the relevant data related to travel time, speed, flow rate, and traffic composition with using a video recording instrumented test vehicle while the roadway geometric features were measured manually. Data for the study were collected on four directional segments of rural two-lane highways with varying geometric features and traffic composition in Johor, Malaysia. Field data obtained were analyzed to estimate the FFS using the two approaches. Results obtained from both methods were compared to ascertain the degree of their consistency or otherwise. Statistical analysis using student t-test indicates that there is no statistically significant difference between the FFS estimates from the two approaches.