Effects of Geometry on Speed-Flow Relationships for Two-Lane Single Carriageway Roads

Speed-flow relationship is usually used as a basis for the performance and capacity analysis of a highway segment. This paper presents the preliminary result of a study carried out to evaluate the effects of road geometry on speed-flow relationship for two-lane single carriageway roads. Speed and traffic volume data at five uninterrupted two-lane single carriageway road segments located in different parts of Malaysia were collected using an automatic traffic counter equipment. Result of the analysis shows that speed and traffic flow variables for a single carriageway road were linearly related. The percentage of no passing zone, road bendiness and hilliness, and the presence of minor junctions were found to have negative effects on the speed-flow curve. The speed-flow-geometry relationship developed in the study produces the estimates of travel speed higher than the values estimated using both USHCM and British models and lower than the MHCM's model. However, more data representing large range of traffic conditions and roadway characteristics are required to enhance the accuracy of the speed-flow-geometry relationship developed in the study.