Are Road Diets Worth It?
Road diets involve narrowing or eliminating travel lanes to make more room for pedestrians and bicyclists. Often they involve downsizing from four lanes to three (two travel lanes and one turn lane). Recently the US Department of Transportation (USDOT) reexamined a multi-state study entitled Evaluation of Lane Reduction "Road Diet" Measures on Crashes. The reexamination uses more advanced study techniques to measure the effectiveness of urban road diets and studies smaller urban areas, like those in Montana.
The theory behind road diets is that, besides making room for non-motorized transportation, they slow traffic to safer speeds and reduce vehicle interaction, thereby reducing crashes. Opponents believe that road diets cause traffic jams, which increase collisions and carbon omissions, eventually forcing people to use alternative routes.
The USDOT reexamination shows that the answer is, as one might expect, dependent on traffic volumes. For roads with annual average daily traffic (AADT) lower than 20,000 vehicles road diets have minimal effects on capacity. For those over 20,000 AADT there is an increased likelihood of congestion, to the point of diverting traffic.
The reexamination also looks at how effective road diets are at reducing collisions. Individual and combined studies show that road diets do have a statistically significant effect on crash reduction. The analysis shows a 29 percent reduction overall in crashes on roads that have been reduced from four to three lanes.
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