Relationship between Horizontal Curve Geometry and Single-Vehicle Crash Occurrence on Rural Secondary Highways

Abstract

Single-vehicle crashes are overrepresented on rural two-lane highway curves. While prior work investigated the relationship between crash occurrence and aggregate curve characteristics, several important aspects related to curve geometry remain uninvestigated. To this end, research was undertaken to evaluate the safety effects associated with several specific horizontal curve-related geometric characteristics (including curve type, curve direction, tangent distance between curves, and curve design speed) on rural two-lane undivided highways, considering non-animal single-vehicle crashes. Eight years of crash data were obtained for 277 mi and 557 mi of curved state and county highway segments, respectively, within Michigan. Several mixed-effects negative binomial models, with county- and site-specific random intercepts, were developed separately for state and county highways. The model results indicated that several key geometric factors were associated with crash occurrence on rural two-lane highways, including curve type, curve direction, tangent distance approaching the curve, inner-curve tangent distance, and curve design speed. Each of the following geometric characteristics was associated with elevated single-vehicle crash occurrence: (1) compound and reverse curves (compared with simple curves); (2) left-turning curves (compared with right-turning); (3) curves with design speeds lower than the speed limit (compared with curves with higher design speeds); (4) longer tangent distances approaching a simple curve or the initial curve in a series; and (5) shorter inner-curve distances between successive compound or reverse curves. Compound or reverse curves should be avoided in favor of simple curves with an elongated inner-curve distance. Enhanced warning signage should be considered where such curve designs cannot be avoided.