The purpose was to (a) determine the horizontal curve features that affect accident experience on two-lane rural roads, (b) determine which types of geometric improvements on curves will affect accident experience, and (c) develop accident reduction factors based on these findings. Very little of this information has been available to highway safety engineers and designers. The results were based on an analysis of 10,900 horizontal curves in Washington State with corresponding accident, geometric, traffic, and roadway data variables. Statistical modelling revealed significantly higher curve accidents for sharper curves, narrower curve width, lack of spiral transitions, and increased super elevation deficiency. All else being equal, higher traffic volumes and longer curves were also associated with significantly higher curve accidents. Ranges of accident reductions for horizontal curves improvements were determined for flattening curves, widening lanes, widening paved shoulders, adding unpaved shoulders, adding a spiral transition, and improving super elevation. From the study findings, a variety of improvements were recommended for horizontal curves with moderate and sharp curvature, and upgrading specific roadside improvements. Expected costs should be compared with estimated accident reductions to determine whether geometric improvements are warranted.
Abstract