Turner-Fairbank Highway Research Center

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Evaluation of Lane Reduction "Road Diet" Measures on Crashes

Product Type

HSIS Summary Report


Persaud, B. and C. Lyon




This Highway Safety Information System (HSIS) summary replaces an earlier one, Evaluation of Lane Reduction "Road Diet" Measures and Their Effects on Crashes and Injuries (FHWA-HRT-04-082), describing an evaluation of "road diet" treatments in Washington and California cities. This summary reexamines those data using more advanced study techniques and adds an analysis of road diet sites in smaller urban communities in Iowa. A road diet involves narrowing or eliminating travel lanes on a roadway to make more room for pedestrians and bicyclists. While there can be more than four travel lanes before treatment, road diets are often conversions of four-lane, undivided roads into three lanes—two through lanes plus a center turn lane. The fourth lane may be converted to a bicycle lane, sidewalk, and/or on-street parking. In other words, the existing cross section is reallocated. This was the case with the two sets of treatments in the current study. Both involved conversions of four lanes to three at almost all sites. 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 first, 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, analysis 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. Because 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.

Link To HSIS Summary Report

Evaluation of Lane Reduction "Road Diet" Measures on Crashes


Accident rates; Before and after studies; California; Countermeasures; Highway design; Highway factors in accidents; Iowa; Lane reduction; Road diets; Traffic accidents; Washington (State)

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HSIS Summaries

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