GEOMETRY-FLOW-SPEED RELATIONSHIPS AND THEIR EFFECT ON LEVEL OF SERVICE

This study deals with two-lane rural highways. Considerable amounts of money are spent yearly on the construction of new such highways and the improvement of existing ones. To ensure that this money is well spent, geometric design standards and flow concepts are used to attain high-speed convenient and safe driving. A certain amount of variance exists, however, among the several standards as well as among traffic flow models. Furthermore, there is considerable lack of knowledge on the relationship between overall geometric measures and traffic-flow characteristics on two-lane rural roads. The speed/flow relationships normally used for rural transportation studies are those derived by the Highway Capacity Manual, which makes no reference to specific geometric parameters. The immediate purpose of this study was to investigate these relationships and, by relating observed speeds to observed flows and road layout characteristics, to derive formulae for predicting the average running speed of vehicles at different levels of traffic flow on two-lane rural highways. For the covering abstract of the conference see IRRD 288720. (Author/TRRL)