Speed metrics and crash risks: Statistical assessment and implications for highway safety policy.

摘要

This dissertation explores the relationships between measures of crash occurrence, the crash rate and the crash density, and various parameters of speed distributions as measured utilizing automatic traffic recorders (ATR) on highways in Iowa, with special attention to the implications of the findings with regard to highway safety policies such as speed limits and their enforcement. The goal of the research was to determine if crash risk is more related to absolute speed or to some measure of variation of the speed distribution. Data on crashes were obtained from the Iowa DOT crash data base. Roadway segments were selected utilizing criteria to avoid problems of over-long sections as were encountered by Solomon in his 1964 report. Aggregated speed metrics were calculated from raw ATR data provided by the Iowa DOT. Visual Basic programs were developed to calculate the basic speed metrics. Standard statistical tests were used to compare the speed distributions as well as their mean and variance. Logistic regression models were developed to explore the relationship between the dependent variable crash probability and the explanatory variables variance, road type, number of lanes, time of day, and day of week. The evaluation included considering two cases, one with all crashes in the segment and the other with weather-related crashes removed. The hypothesis, that one or more of the speed metrics could be used to determine crash risk, was not supported by the results of the analyses. Recommendations for further research include utilization of new technology, such as (in-vehicle) event data recorders and passive speed measuring devices, to collect non-aggregated speed data.