League of Michigan Bicyclists / Flickr

A closer look at what's been called "one of the transportation safety field's greatest success stories."

Earlier this week, the U.S. Department of Transportation announced an 18-month campaign to improve road safety across the country. One of the things DOT plans to do is create a guide to "road diets" that it will distribute to communities and local governments. DOT says that road diets can reduce traffic crashes by an average of 29 percent, and that in some smaller towns the design approach can cut crashes nearly in half.

But what exactly is a road diet? A good place to start is the apparent source of DOT's safety figures: a 2013 white paper on road diets prepared for the Federal Highway Administration by Libby Thomas of the University of North Carolina Highway Safety Research Center. While road diets have been around for decades, writes Thomas, it's only in the past 10 years or so that experts have understood just how beneficial they can be:

Road diets can be seen as one of the transportation safety field's greatest success stories.

The concept of road diets emerged as a response to a common practice of expanding two-lane urban arterials into four lanes once vehicular traffic hit a certain point—roughly 6,000 cars a day by some estimates. The original thinking held that wider roads meant better traffic flows, especially at rush-hour, but new lanes also attracted new traffic, and outside the peak periods you'd end up with lots of wasted road space. An analysis of road widening in the small city of Fort Madison, Iowa, showed an increase in traffic volumes, but also delay, speed, and crash and injury rates:

Welch (1999), TRB Circular E-C019

Realizing these unintended outcomes, some localities implemented a type of road diet: reconfiguring the four lanes (two in each direction) into three (one each way plus a shared turn lane in the middle). The change dramatically reduced the number of "conflict points" on the road—places where a crash might occur. Whereas there might be six mid-block conflict points in a common four-lane arterial, between cars turning and merging, there were only two after the road diet:

Iowa Department of Transportation (2001)

Likewise, at an intersection, eight potential conflict points became four after a road diet:

Iowa Department of Transportation (2001)

The result was a much safer road. In small urban areas (say, populations around 17,000, with traffic volumes up to 12,000 cars a day), post-road diet crashes dropped about 47 percent. In larger metros (with populations around 269,000 and up to 24,000 daily cars), the crash reduction was roughly 19 percent. The combined estimate from all the best studies predicted that accidents would decline 29 percent, on average, after a four-to-three-lane road diet—DOT's reported figure.

These benefits alone would be enough to merit more road diets, but there were plenty of others. Bicycle and pedestrian traffic tends to soar at these sites, as the recaptured road space gives way to bike lanes or street parking that provides a sidewalk buffer from moving traffic or crossing islands, and as vehicle speeds decline (especially for high-end speeders going more than 5 miles per hour over the limit). Traffic volumes, meanwhile, typically stay even in such a corridor: some drivers diverted to other parts of the street network, while the rest quickly soak up any vacated space.

Best of all, these kinds of changes don't cost much. When timed with regular road maintenance and re-paving, road diet policies require little more than the paint needed to re-stripe lanes. They're about as cheap and cost-effective as infrastructure improvements get, which has led some to wonder why the technique isn't used more widely; here's planner Charles Marohn writing earlier this year at Strong Towns:

Why, when our leadership has expressed so clearly the enormous financial gap we have in funding a "world class" transportation system, are road diets not an obsession of transportation departments everywhere?

One source of the hesitation (aside from general car reliance) may be that the evidence suggests caution when implementing road diets on corridors that carry more than 20,000 cars a day. For sure, some major urban roads can't slim down overnight without creating huge traffic problems. But road diets have also worked in New York City of all places: a 2013 study found significant crash reductions across treated sites.

And improvements can be made even when lanes aren't removed. The NYC DOT recently reported that traffic flows remained strong while safety increased when traffic lanes were narrowed to accommodate bike lanes. That approach might need its own name—call it a lane diet, maybe—but the outcome seems to be the same.

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