Modeling Safety Effects of Geometric Design Consistency on Two-Lane Rural Roads Using Mixed Effects Negative Binomial Regression

Previous research has examined the relationship between roadway safety and design consistency using measures such as the difference between design and operating speeds and changes to driver workload. While such measures have proven effective in identifying inconsistences in the roadway, they do not directly identify the conditions associated with safety performance. The purpose of this research was to directly quantify the effects of geometric design consistency on roadway safety using measures that can be linked to specific geometric elements. Using mixed effects negative binomial modeling, two safety performance functions (SPFs) were developed. The first contains typical roadway parameters that are suggested for use by several contemporary safety management tools, while the second contains additional geometric design consistency parameters. After Empirical Bayes adjustments were performed on 5 years of crash data from over 2,100 fixed-length (2.5 miles) segments of two-lane rural roadway, the sites with promise (SWiPs) were identified. The disparity between SWiPs identified by the two SPFs was evident; 40 unique sites were identified by each model out of the top 220 sites identified. Along with a marked shift of rankings, this constitutes a 19 percent change in the top 10 percent of sites.