Crash Modification Functions for Rural Skewed Intersections

Abstract

This study evaluated the safety influence of intersection skew angle on rural two-lane, two-way facilities by calibrating crash modification factors. Ten years of crash history among federal aid and nonfederal aid highways was used to develop crash modification functions at three-leg and four-leg stop-controlled intersections. Skew angle was investigated as a parameter in the safety performance functions models both as a continuous variable, with observed values ranging from zero to 80°, and categorized into ranges. A few transformations of the skew parameter were considered, such as the flexible-form model having skew interaction with annual average daily traffic, and a Hoerl curve. Both three-leg and four-leg intersections exhibited an initially increasing trend of crash rates followed by a decreasing trend as the skew angle increased. A categorical model was found to best describe the skew relationship using discrete skew angle ranges. Among three-leg intersections, a skew angle between 17° and 27° experienced 22% more crashes than perpendicular intersections. However, more highly skewed three-leg intersections exhibited a decreasing relationship to increasing skew angle. Among four-leg intersections, a skew angle between 17° and 27° experienced 40% more crashes, whereas intersections with a skew angle greater than 45° did not have significantly different crash occurrence than perpendicular intersections. The implications of assuming a monotonic increasing relationship to skew angle are challenged as a result of this study.