HOW MUCH DO YOU LOSE WHEN YOUR ROAD GOES ON A DIET

Transportation engineers and planners may implement road diets with the objective of reducing vehicle speeds and motor-vehicle crashes and injuries. Typical road diets consist of converting four-lane undivided roads into three lanes (two through lanes plus a center turn lane) with the remaining space used for bicycle lanes, sidewalks, and /or on-street parking. This study investigated the actual effects of road diets on motor-vehicle crashes and injuries using data from cities in California and Washington state. A and analysis using a yoked study design of the road diet and comparison sites indicated that the percent of crashes at the road diet sites during the after period was slightly lower than at the comparison sites. Further analysis using a negative binomial model controlling for possible changes in average daily traffic, study period, and other factors indicated no significant treatment effect. Crash severity was virtually the same at road diet and comparison sites. However, there were differences in crash type distributions between road diet and comparison sites but not between before and after periods. Implementation of a road diet should be made on a case-by-case basis where traffic flow, vehicle capacity, and safety are all considered. Also, the effects of road diets should be evaluated further under a variety of traffic and roadway conditions.