Safety Benefits of Spiral Transitions on Horizontal Curves on Two-Lane Rural Roads

Abstract

A vehicle entering or departing a horizontal curve must safely undergo a change in steering angle and a resulting change in side friction forces. The safety-related effects of two alternative designs were evaluated for the “transition” sections of pavement where the curve and tangent meet— a spiral transition, where the tangent and curve are linked by an ever-tightening spiral curve, and a nonspiral transition where the tangent directly abuts the horizontal curve. Using crash and roadway inventory data from over 15,000 transition sections (curve ends) in the state of Washington, contingency table analyses and linear logistic modeling were conducted to explore differences in the probability of one or more crashes on the two types of transitions within each of three types of terrain—level, rolling, and mountainous. At level terrain sites, spiral presence is related to a decrease in crash probability on curve ends with degree of curve greater than approximately 3 degrees. For rolling terrain, a decrease was predicted for all sites on minor arterials and major collectors, and for principal arterial sites with average daily traffic greater than 3,700 vehicles per day. In contrast, in mountainous terrain, an increase in crash probability due to the spiral was predicted for all sites except those with both wider roadway widths and wider shoulders. The models for the mountainous terrain sites did not fit the data as well, indicating the presence of other unmeasured confounding variables.