Effects of Operating Speed and Traffic Flow on Severe and Fatal Crashes using the U.S. Road Assessment Program Methodology and Field Data Verification

This paper explores the effects of operating speed and traffic flow on roadway safety in light of the methodology provided by the U.S. Road Assessment Program (usRAP). Unlike traditional approaches, usRAP produces a systemic expected roadway safety performance, more specifically the likelihood of being involved in a severe or a fatal crash, that is derived purely from roadway, roadside, and traffic characteristics, without need for detailed historical crash data. Data from over 7,000 mi of segments coded using the usRAP protocols and 5 years of crash data were used to examine changes in expected safety performance with changes in operating speed and traffic volumes. Speed and flow emerged as candidates for initial exploration as their effect is explicitly considered in the usRAP formulation for all crash types. The usRAP methodology indicated a gradual increase in the frequency of expected severe and fatal crashes with an increase in the operating speed, and such trends followed those observed in the field. Increase in traffic flow was generally associated to increase in severe and fatal crashes, but to a much smaller scale compared with the effect found for speed. Effects of traffic flow were more evident at smaller ranges, both in the field and in the usRAP results, with the safety effects diminishing and even reversing as the flow approached lane capacity. Crash data were examined using a risk ratio that considers the relative frequency of severe and fatal crashes to the exposure of a given segment group, as well as star rating scores and star ratings from usRAP outputs.