America's roundabout capital sees big reductions in crashes

Over the last few decades, Carmel, Ind., has become the roundabout capital of the U.S. It has more than any other American city at a total of 138. And according to a recent IIHS study, those roundabouts have made a pretty dent in crashes: 47% overall across all types.

The study looked at crash data for the two years before an intersection was changed over to a roundabout, and then the two years after. This was done with 64 roundabouts in the city including single-lane, multi-lane, and the "double-teardrop" or "dogbone" roundabout, which is like a normal one, but with a stretched, narrow middle. The dogbone versions saw the biggest improvements with a reduction in all crashes by 63%, and a reduction in injury-causing crashes by 84% compared with what would've been expected in conventional traffic light- or stop sign-controlled intersections. Multi-lane roundabouts didn't show a huge change, with damage-only crashes going up slightly, and injury-causing crashes dropping slightly.

The IIHS believes that part of the reason the dogbone roundabouts saw such big improvements was because of their use at high-speed intersections. They've been added to areas where four-lane parkway roads cross with smaller surface streets. And the improvements across the board likely come from the fact that roundabouts both slow vehicles and reduce the chances for head-on or right-angle collisions. As for the limited effect of multi-lane versions, the IIHS believes some of it comes from the design being a bit difficult or confusing for drivers, but notes that some studies show drivers get used to them and see reductions in crashes over time.

Besides the safety benefits, Carmel's roundabouts have improved traffic. On the higher-speed parkway, plans had been made to expand from four lanes to six. Instead, the city added the dogbone roundabouts. The city has reported less congestion, and this is on top of the fact that the city has doubled in population since the earliest parts of the study to the latest. So overall, it seems that Carmel's bets on roundabouts are paying off big time.

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