An operational and safety evaluation of alternative horizontal curve design approaches on rural two-lane highways

This paper evaluates the effects of superelevation on 85th percentile speeds and accident experience as well as the effects of side friction demand on accident experience at horizontal curves on rural two-lane highways. These evaluations were conducted as part of research toward development of a design consistency evaluation model for the United States. The operating speed analysis verified previous models that used degree of curvature, length of curve, and deflection angle as independent variables for estimating 85th percentile speed on curves. The analysis also found superelevation to be a statistically significant independent variable. Independent variables in the accident analysis included degree of curvature, operating speed reduction, superelevation deficiency, and implied side friction demand. Operating speed reduction and superelevation deficiency were found to be significant accident predictors; however, implied side friction demand was the strongest accident surrogate. Comparisons of alternative horizontal curve design methods, with respect to which speed should be used for the design of curves, were made. The 85th percentile speed on a curve was the strongest performer of four curve design ideologies and is recommended for use in horizontal curve design. Superelevation has significant effects on 85th percentile speed on rural two-lane horizontal curves. Operating speed reduction, superelevation deficiency and side friction demand based on 85th percentile operating speeds have significant effects on the safety of horizontal curves. These findings provide further support of the adoption of an operating-speed based design procedure for two-lane rural highways in the United States.