Evaluating speed consistency between successive elements of a two-lane rural highway

The V85 speed reflects the 85th percentile speed of vehicles in a traffic stream passing a given element on the highway section. The large amount of speed differential in V85 between the two successive elements along a highway reflects lack of speed consistency in the highway section. This lack of speed consistency can result in increased crash risk. Recent research points to several problems associated with the conventional method of measuring ΔV85 between successive highway elements: (1) failure to reflect individual vehicle speed profiles; and (2) failure to account for inter-dependence in individual vehicle speeds between successive elements. These problems have serious implications for justifying safety treatment when conventional ΔV85 measure is applied. A number of researchers have suggested that the estimated speed differential based on individual vehicle speed profiles in successive elements is significantly higher than that obtained using the conventional approach. In this paper, we assess the safety implications of using the conventional ΔV85 and introduce a hierarchical model for considering individual vehicles speed consistency. These findings lead to important implications for introducing engineering treatments to improve safety along in two-lane rural highways based on the criteria of speed consistency.