This research aimed to study consistency of design on 2-lane rural highways and ascertain the nature of a relationship between consistency and safety level. The immediate goals 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 2-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 2 measures, thresholds quantifying the design quality were suggested. Based on the 2 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 2-lane highways. This was done initially on a limited data set of 9 local, 2-lane highway sections. It was found that as design consistency increased, crash rates decreased significantly. In a 2nd phase, the analysis was expanded and the same consistency model was applied to a data set of 28, 2-lane U.S. highways. It was found that crash rates decreased when the consistency value increased.
Abstract