The safety effects of speed limit changes: use of panel models, including speed, use, and design variables. Transportation Research Record

This work estimates the total safety effects of speed limit changes on high-speed roadways using traffic detector data and Highway Safety Information System (HSIS) data from 1993 to 1996. In order to gauge the total effects, this study applies a sequential modeling approach wherein average speed and speed variance models are first estimated, based on roadway design, use and speed limit information. Then, crash counts (of varying severity) are estimated, based on the speed estimates, design, and use variables. The four years of data come from 63,937 “homogeneous ” roadway segments along 7 interstates and 143 state highways in Washington State. A random-effects negative binomial model was selected among several alternative panel and non-panel models for count data. Results indicate that the average road segment in the data set can be expected to exhibit lower non-fatal crash rates up to a 55 mph (88 km/h) speed limit. In contrast, fatality rates appear unresponsive to speed limit changes. Fatal and non-fatal rates fall for design reasons, including wider shoulders and more gradual curves, which appear to be key design variables. However, fatal and non-fatal rates move differently when traffic levels rise, with non-fatal rates remaining unchanged and fatal rates falling