Improved speed-profile model for two-lane rural highways

The speed-profile model has been suggested as a valuable tool for evaluating geometric design consistency for two-lane rural highways with isolated and combined horizontal and vertical alignments. The model determines the operating speeds on the speed-change (SC) segment, which is the distance between speed-limiting curves. The speed-limiting curves are the horizontal curves and the limited sight-distance crest vertical curves on horizontal tangents, where the sight distance required by the design guides is not satisfied. The model assumes that deceleration begins where required, which implies that the next curve is visible when deceleration starts. This paper presents an extension to the speed-profile model to incorporate the effect of sight obstruction on operating speeds and deceleration rates. The SC segment may include nonlimited sight-distance crest vertical and sag vertical curves. These curves may present sight obstruction. A procedure to determine whether the sight line is obstructed is developed. If it does, simple formulas are applied for revising the operating-speed profile. The extended model is suitable for inclusion in the design consistency module of the interactive highway safety design model.