Evaluation of Alternative Horizontal Curve Design Approaches on Rural Two-Lane Highways (TTI-04690-3)

This report documents evaluations of the effects of superelevation on operating speeds and accident experience and the effects of side friction demand on accident experience at horizontal curves on rural two-lane highways. The goal of the evaluations was to glean insights about the most appropriate speed-related assumptions for alignment design.Variables considered in the operating speed analysis included degree of curvature, length of curve, deflection angle, and superelevation. Two previously developed regression models for estimating 85th percentile speeds on curves were verified. Superelevation was a statistically significant predictor of 85th percentile speeds on horizontal curves when added to these models.Potential accident surrogates examined included AADT, length of curve, degree of curvature, lane width, lane plus adjacent shoulder width, total pavement width, operating speed reduction, superelevation deficiency, and implied side friction demand. The cross section width variables were not significant in this study, although some trends in the data were noted. Operating speed reduction and superelevation deficiency were statistically significant accident predictors, and implied side friction demand was the strongest accident surrogate found.Comparisons of alternative horizonal curve design methods, with respect to the speed that should used for curve design, were made. Eighty-fifth percentile operating speed on the curve was the strongest performer of four basic curve design ideologies. These findings support the adoption of an operating-speed-based design procedure for rural two-lane highways in the United States.