Development of Rate-Based Statewide Safety Performance Measures of Work Zones Using Imperfect Exposure Data: A Virginia Case Study

Ideally, state departments of transportation would track work zone crashes over time while simultaneously accounting for traffic exposure using work zone vehicle miles of travel (VMT). Work zone VMT is often difficult to obtain, however, because of inconsistent logging of work zone locations and changes in traffic volumes during construction. As a result, the Virginia Department of Transportation was interested in developing reliable work zone safety performance measures that could account for exposure using data that were readily available to them from existing sources. This study developed statewide rate-based measures for Virginia by combining two data sources: the crash database and the VaTraffic database containing information on roadway activities such as work zones. Data from 2009-2013 were prepared for interstate and non-interstate highways at four temporal levels. A linearity condition, with the numerator and denominator of a rate being proportional, was used to develop recommended work zone safety performance measures. The analysis revealed several reliable rate-based measures that did not rely on having accurate work zone VMT. Rates based on the exposure reflecting all three output-aspects of work zones (count, duration, and length) were better than those reflecting two of the three. It is worth noting that the study conclusions are valid for statewide annual safety performance measures and may not be valid for measures at different geographical and/or temporal levels. Although better results could likely be achieved if reliable work zone VMT were available, the developed measures appear to represent an improvement over pure crash count measures.