Effect of Stopping Sight Distance on Crashes at Crest Vertical Curves on Rural Two-Lane Highways

Abstract

This paper demonstrates that the crash reduction benefits of improving stopping sight distance (SSD) vary substantially over the highway system, with the greatest benefits likely at locations where a horizontal curve, intersection, or driveway is in a sight-restricted area. The paper analyzes crash data for 452 crest vertical curves on two-lane highways in the state of Washington, approximately equally distributed between sites with SSD below and above existing design criteria. Preliminary analyses found statistically significant differences in crash frequency between the groups of sites in SSD categories, but the statistical significance of these differences disappeared when a factor was introduced to account for the presence of horizontal curves, intersections, or driveways that were hidden from an approaching driver's view by the sight restriction. The presence of such hidden features was found to be highly statistically significant. There is a need for highway design guidance to give higher priority to addressing sight distance limitations when hidden roadway features are present and to recognizing that sight distance improvements may have little effect on crash frequency or severity when such hidden features are not present. Consideration should also be given to development of SSD models for use in highway design to reflect better the differences in SSD needs on various elements of the roadway system.

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