Evaluation of Lane Reduction "Road Diet" Measures on Crashes

Highway Safety Information System (HSIS)

While potential crash-related benefits are cited by road diet advocates, there has been limited research concerning such benefits. Two prior studies were conducted using data from different urbanized areas. The firsst, conducted by HSIS researchers, used data from treatment sites in eight cities in California and Washington. The second study analyzed data from treatment sites in relatively small towns in Iowa. While the nature of the treatment was the same in both studies (four lanes reduced to three), the settings, analysiss methodologies, and results of the studies differed. Using a comparison of treated and matched comparison sites before and after treatment and the development of negative binomial regression models, the earlier HSIS study found a 6 percent reduction in crash frequency per mile and no significant change in crash rates at the California and Washington sites. Using a long term (23 year) crash history for treated and reference sites and the development of a hierarchical Poisson model in a Bayesian approach, the later Iowa study found a 25.2 percent reduction in crash frequency per mile and an 18.8 percent reduction in crash rate. Beacause of these differences, researchers from the National Cooperative Highway Research Program (NCHRP) 17-25 project team obtained and reanalyzed both data sets using a common methodology. This summary documents the results of that reanalysis.

Filed in: Engineering, Crashes and Safety

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