Keith Knappkknapp@iastate.edu email >
About the research
Ninety-four traffic safety projects were analyzed to determine crash reduction factors and benefit/cost ratios for seven different improvement categories. Overall analysis showed that the projects had a mean crash reduction rate of 23 percent. In all cases, the 90 percent confidence interval (the interval at which we can be 90 percent confident that the true mean lies within) is positive. Adding turn lanes while modifying the signal phasing (i.e., adding left-turn arrows) had the highest crash reduction factor. Replacing pedestal-mounted signals with mast arm mounted signals also had a significant effect. This research also analyzed benefit/cost ratios of improvement projects. For all types of projects, a mean benefit/cost ratio of 6.3 was determined. Pedestal mount replacement had the highest benefit/cost ratio of all the project types. There were only two projects in the category of adding turn phasing to the existing signals; therefore, the ratio and confidence interval are not of much value. Analyzing crash data is a very complex task. It has been generally known that making improvements of a certain type (e.g., adding a traffic signal) will oftentimes change the type of crashes rather than simply reduce the number of crashes. Therefore, just because a particular type of improvement shows that crashes may increase, a closer examination should be made into the type of crashes that are increasing and their severity. In the cases above where the confidence interval included a negative number, the sample sizes are very small. More data should be collected before making any strong conclusions about those two particular types of projects.