ESTIMATION OF SAFETY EFFECTIVENESS OF COMPOSITE SHOULDERS ON RURAL TWO-LANE HIGHWAYS

Abstract

A paved shoulder has been regarded as an effective safety improvement to reduce crashes. There is belief that there is a diminishing safety benefit for each additional increment of paved shoulder width. Thus there may be opportunities for greater system-wide safety benefits from paving longer roadway segments with a composite shoulder than paving shorter roadway segments with a full-width paved shoulder. The objective of this study was to determine the safety benefits of composite shoulders - such as a small paved shoulder combined with turf outside of that. This approach was part of the Kansas Department of Transportation's effort to find “Practical Improvements” to maximize benefits relative to the input costs required. Among the 8,300 miles of rural two-lane highways in Kansas, approximately 25 percent of them are equipped with composite shoulders consisting of three ft of pavement with the remainder turf. Their safety effectiveness was studied using the Empirical Bayes (EB) approach and the cross-sectional approach. Three developed Safety Performance Functions (SPFs) were used to create Kansas-specific Crash Modification Factors (CMFs) for composite shoulders compared with segments with no or unpaved shoulders. It was found that upgrading narrow unpaved shoulders to composite shoulders can reduce shoulder related crashes by up to 61 percent and fatal and injury crashes by 31 percent. Based on these results, 20-year projections were developed projecting the safety benefit that can be achieved through implementing these safety improvements.