70-mph speed limit and motor vehicular fatalities on interstate highways

In May 1996, Alabama increased the rural interstate highways speed limit to 70 mph. This study was conducted to determine if the faster speed limit had any influence on the number of motor vehicular crash (MVC) deaths. The null hypothesis was that no significant change occurred in the number of deaths since the faster speed was introduced. Using MVC fatality data collected by the Alabama Department of Public Safety and published by the state of Alabama from 1984 to 1999, a time-series study designed was used to examine the number of MVC deaths before and after the speed limit change. The trend of MVC deaths was examined for both rural and urban interstate highways and those occurring on Federal and state highways. There were 174 interstate MVC deaths in 1997 and 165 in 1999; both figures represent a significant increase (P <.05) from the trend of previous years' results. State and Federal highways demonstrated no significant change. By 1998 the number of interstate highway MVC deaths dropped to 114, falling back within the expected trend established by the results of the previous years. The number of rural interstate MVC deaths was found to be significant in 1997 and 1999. The faster speed limit was associated with an increase in MVC deaths for the first year after the speed limit increase. The decline in the number of MVC deaths for 1998 was unanticipated. There are many other factors that need to be evaluated before determining the cause and effect that higher speed limits have on MVC fatalities.