The term "road diet" usually refers to converting a road from four lanes to three lanes, with one through lane in each direction and a two-way continuous left-turn lane. In Iowa, 15 of these conversions have been completed. This article analyzes the safety impacts of these conversions. A full Bayes approach and a classical before-and-after study with yoked comparison sites are used. The two study methods produced similar results. Findings showed significant reductions in the crash frequency per mile, crash rate, number of injury crashes and crash severity. Reductions were also shown in the crash involvement rate for drivers aged 25 and younger and 65 and older. Significant reductions were also found in the number of crashes related to left turns and stopped traffic. These results confirm the benefits of converting four-lane undivided roadways to three-lane cross sections in selected locations.
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