This report studies the impact of the 65-mph speed limit on Interstate highway fatalities using the Box-Jenkins model and Box-Tiao intervention analysis. An extensive survey was conducted to collect accident information individually from each state, including the accident frequency during the 2-year before (1985-1986) and the 2-year after (1988-1989) periods. With several follow-up phone calls, the final survey results yield an accident data set for twelve 65-mph and two 55-mph states. This report summarizes the research results in the following sequence: Comparison of the accident frequency before and after the speed limit change in both 65-mph and 55-mph states; Assessment of the accident rate before and after the speed limit change in both 65-mph and 55-mph states; Evaluation of the long-term nationwide fatality pattern on rural Interstate highways; and Estimation of the 65-mph speed limit impact with intervention analysis. The report is organized as follows: Chapter 1 is an introduction. Chapter 2 describes the characteristics of data sets used for accident and fatality analyses. Preliminary results based only on the numerical change in fatalities and accidents between the before and after periods are also presented in this chapter. Chapter 3 first introduces three statistical tests for use in assessing the speed limit impacts on accidents, followed by a detailed discussion of the test results. Concluding comments with respect to the data quality and reliability of the research findings are also included. Chapter 4 investigates the speed limit impact on Interstate highway fatalities. A time-series model based on the Box-Jenkins method is presented along with some statistics for identification of any systematic trend in the fatality patterns. The standard intervention analysis is then applied in conjunction with the time-series model to test the statistical significance of the 65-mph speed limit on fatalities. The overall research findings and future research needs are summarized in Chapter 5. (A)
Abstract