Safety Impacts of Highway Shoulder Attributes in Illinois

Abstract

The single greatest category of highway vehicle crashes is run-off-the-road incidents. Significant material differences and elevation changes in shoulder edges pose a potential safety hazard when a vehicle leaves the travel way. Shoulder paving is recognized as a positive countermeasure to reduce a shoulder drop-off hazard. This report documents an in-depth analysis of safety impacts of shoulder attributes using data on Illinois statemaintained highways for period 2000-2006. Preliminary data analysis is first conducted to establish the correlation between shoulder-related crashes by type and severity category and shoulder attributes such as shoulder material type and outside paved shoulder width. Then, an analytical procedure is developed and applied to the Illinois data for assessing safety impacts of shoulder paving and prioritizing highway segments for shoulder paving using the Empirical Bayesian (EB) analysis and cross-sectional analysis approaches and an optimization model. While this procedure does not eliminate the need for human judgment, it could help experts make better decisions using optimization techniques. The findings are expected to help Illinois Department of Transportation (DOT) update current design manuals.