Two-Lane Rural Highways

Abstract

A series of investigations was performed in Germany to understand traffic operations on two-lane highways and to make the experiences accessible for traffic performance analysis in practice. As an initial step, traffic flow on several sections of rural two-lane highways was observed. The observations led to a set of empirical speed–flow relationships for highways of different characteristics. On the basis of these results, a microscopic simulation model was calibrated. The model was able to produce a comprehensive set of speed–flow diagrams for all kinds of rural two-lane highways. The diagrams revealed several unexpected properties: a concave shape and nonlinear influences of the external geometric parameters such as curvature or gradients. The simulated results were then approximated by a simple quadratic equation with the parameters depending on geometric conditions and the percentage of heavy vehicles. Thus, the speed–flow diagrams were obtained for the German Highway Capacity Manual. In this case density was used as a measure of effectiveness for the definition of levels of service. This guideline has been used successfully for 3 years.