Road Diets Safety
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Road Diet Safety
A “road diet” is converting a roadway from four lanes to three lanes (one through lane in
each direction and a two-way, continuous left-turn lane), is frequently suggested as a traffic
calming solution or to address left-turn related crashes on undivided four-lane urban roadways
where widening may not be an option (Wikipedia, 2006).
A data analysis to assess the reduction in crash history due to “road diets” in Iowa was conducted by Iowa State University (ISU) in cooperation with Iowa Department of Transportation Office of Traffic and Safety (TAS). The study utilized monthly crash data and estimated volumes obtained from TAS for 30 sites, 15 treatments and 15 controls, over 23 years (1982-2004). The sites had volumes ranging from 2,030 to 15,350 during that time span and were largely located in smaller urbanized areas.
The main research objective was to assess whether “road diets” appear to result in crash reductions in Iowa roads. To meet the objectives ISU and TAS analyzed crash data at each site before and after the conversions were completed. Given the random and rare nature of each crash event, ISU and TAS fitted a hierarchical Poisson model to crashes; where the logged mean was expressed as time period, seasonal effects, and a random effect corresponding to each site. Estimation of the model parameter was conducted and the results indicated a 25.2 % reduction in crash frequency per mile and an 18.8% reduction in crash rate. This differs from a previous, much publicized study, which reported a 6% reduction in crash frequency per mile and an insignificant indication for crash rate effects. The results from the Iowa study fit practitioner experience and agree with another Iowa study utilizing a simple before/after approach on the same sites. The monthly crash data clearly showed seasonal effects; hence, the four seasons were described as: winter (December Ð- January), spring (March Ð- May), summer (June Ð- August), and fall (September Ð- November). To account for seasonality in crash rate, ISU and TAS included three smoothly evolving