Spatial evaluation of speed-flow-geometry relationship on two-lane rural highways

The mean travel speed of drivers on uninterrupted flow facilities such as two-lane rural highways is deemed as good performance indicator for the subject road class; as the variable relates well with user perception. However, the operating conditions on two-lane roads relating to travel speed is different from those on other types of facilities, as fast moving vehicles in either direction are usually impeded by slower moving ones in the same travel direction and also facing oncoming traffic in the opposing lane. Thus, impeded vehicles may be compelled to travel at lower speeds than desired; particularly, in the absence of sufficient sight distance and permissible gap in the opposing traffic stream, being the appropriate lane used for passing maneuvers. This implies that the operating speed on two-lane highways substantially depends on the level of traffic flow and perhaps, its composition as well as the roadway geometric features. This paper examines the effects of traffic level, composition and road geometric features on the operating speed on two-lane highways based on a spatial approach. An empirical model relating mean travel speed to traffic flow parameters and highway geometric features was derived for prediction of mean travel speed on two-lane rural highways based on easily observable variables

Free flow speed on two lane rural highways: an empirical evaluation of estimation models

Free-flow speed (FFS); a fundamental parameter in the analysis of two-lane highways, is either estimated through direct field measurement or using analytical model. Regarding the latter 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 proportion of motorcycles in the traffic stream. Likewise, the Highway Capacity Manual (HCM) suggested an approach for direct field measurement of FFS; preferably, at a combined two-way flow rate not in excess of 200 veh/h. For most two-lane highways, observing a two-way flow rate of 200 veh/h or less is rarely possible. In such a case, the HCM recommended that mean speed could be observed at flow rates higher than the limiting value and adjusted accordingly based on an analytical model provided by the manual. This paper presents an empirical appraisal of the MHCM and HCM – FFS estimation models. Data for the study were collected from four directional segments of rural two-lane highways in Johor, Malaysia. A video recording intrumented test vehicle was used for observing the relevant variables needed in this study. An analysis of the obaseved data resulted in mean FFS values of 83.35 km/h and 83.03 km/h from HCM and MHCM models, respectively. FFS estimates from the two data sets were compared to ascertain the degree of their consistency or otherwise using statistical analysis. Results from statistical analysis using t-statistic at 0.05 significance level indicates that there is no statistically significant difference between the FFS estimates from the two approaches. Thus, finding from this study suggests that either of the two approaches could be used for measuring FFS; particularly, at combined two-way flow rates exceeding 200 veh/h