New Consistency Model for Rural Highways and Its Relationship to Safety

Abstract

The goal of the research was to study consistency of design on two-lane rural highways and to ascertain the existence of a relationship between consistency and safety level. The immediate objectives were to develop new, independent measures of consistency that could reflect the similarity (or lack thereof) of performance along an entire level or hilly section, to develop a new consistency model, and to find the relationship between the new model and crash rates on two-lane rural highways. Two consistency measures were developed: the first was the relative area bounded by the speed profile and the average weighted speed; the second was the standard deviation of operating speeds in each design element along the entire section investigated. Following an extensive sensitivity analysis of these two measures, thresholds that quantified the design quality were suggested. Based on the two independent measures, a consistency model was developed; and thresholds for good, acceptable, and poor design consistency of any section were proposed. Additional analysis was conducted on the relationship between the proposed consistency model and the safety level of two-lane highways. This was done initially on a limited data set of nine local, two-lane highway sections. It was found that as design consistency increased, crash rates decreased significantly. In a second phase, the analysis was expanded and the same consistency model was applied to a data set of 28 two-lane U.S. highways. It was found that crash rates decreased when the consistency value increased.