Time Series Analysis of Macrolevel Annual Safety Performance Measures for California
Authors:Kareem Alhassen, Wen Cheng
Mentor:Wen Cheng, Professor, Cal Poly Pomona
Municipal, state, and federal agencies in the United States that are responsible for traffic safety have used crash rates such as fatalities per 100 million vehicle miles traveled (VMT) as traffic safety performance measures. However, the appropriateness of using such rates as performance measures has not been examined empirically, although the rates have been made public. This study examined 20 candidate crash rates (e.g., fatalities per million population and injury crashes per million registered vehicles) for an annual safety performance measure for the State of California by using empirical data from 2001 through 2010. The collision data were collected from three major sources: the California Office of Traffic Safety (OTS), the Statewide Integrated Traffic Records System (SWITRS), and the Federal Census. To consider the serial correlation among crash data, the research used a linear regression model with autoregressive error term to relate a crash or victim count with an exposure over time. The study found that the injury rate per driver and the crash rate per VMT seem appropriate as, respectively, long-term (2001 to 2010) and shorter-term (2005 to 2010) safety performance measures for the State of California. Statistical uncertainty should be considered when these rates are used to measure safety performance.