Improved Volume–Delay Function for Two-Lane Rural Highways with Impact of Road Geometry and Traffic-Flow Heterogeneity

Abstract

Two-lane rural highways represent the absolute majority of road networks globally, but there is a noticeable lack of research on this type of road for traffic planning purposes. This paper presents the relationship between average flow speed on two-lane rural highways and traffic volume and composition, and roadway horizontal and vertical geometric characteristics, expressed in the form of a volume–delay function. A new volume–delay function was developed through a combination of field research and traffic microsimulation. Field research was conducted on the main road network in Bosnia and Herzegovina. In total, more than 1 million simulations of different traffic situations were performed in an automated process. The results showed that a combination of traffic composition and geometric features (bendiness and longitudinal grade) has a crucial impact on average flow speed. The new proposed model predicts average flow speed in various traffic situations with high reliability (R2=0.77), compared with R2=0.26 for the widely used Bureau of Public Roads (BPR) volume–delay function.