How curve geometry influences driver behavior in horizontal curves - A study of naturalistic driving data

Abstract

Traffic accidents are commonly found on horizontal curves. It is therefore important to design horizontal curves that elicit safe driving behavior. This thesis investigates how curve geometry influences the driver behavior in horizontal curves by using naturalistic driving data. Seven curves were selected, all on rural two-lane roads with a posted speed limit of 70 km/h. The curve geometry factors studied were radius, presence and lengths of spiral transitions and lengths of approach and exit tangent. Regression analyses were used to analyze how these factors influenced the driver behavior in terms of speed and maximum lateral acceleration. The speed behavior was also analyzed by studying speed profiles. The study showed that all studied factors influenced the driver behavior in the selected curves. The speed increased for larger radii, and longer approach tangent resulted in higher speed at the beginning of the curvature. Longer transition resulted in higher speeds at the center of the curve, and did not affect the maximum lateral acceleration – indicating a changed trajectory. All but one of the selected curves did not fulfill the existing recommendations on curve design provided by the Swedish Transport Administration. The speed profiles showed that the median speed exceeded the posted speed limit in all curves except one. Driving at higher speeds than what a curve is designed for could increase risks. The study also found that speed patterns in curves seemed to be independent of the choice of speed.