An innovative double-teardrop roundabout design installed in Carmel, Indiana, cut injury crashes by 84% and reduced all crashes at multiple locations by nearly two-thirds, according to an analysis by the Insurance Institute for Highway Safety. 

The double-teardrop was installed along Keystone Parkway and at other busy intersections. Experts say the design is remarkably effective largely because it can be installed at the most dangerous intersections. 

Carmel has over 100 roundabouts — more than any other city in the United States. 

To determine how effective roundabouts actually are in Carmel, IIHS researchers examined crash data from 64 roundabouts over the two years before and after their construction. 

The sample included 21 single-lane, 10 double-teardrop, and 33 other multilane roundabouts built between 2005 and 2017. Before conversion, 25 of the intersections were controlled by traffic signals, 19 were four-way stops, four were three-way stops and 16 were two-way stops.

The findings: The conversion to roundabouts — of all types — resulted in a 47% reduction in injury crashes overall, compared with the number the researchers projected would have occurred if roundabouts had not been adopted.

Digging deeper, the researchers found that single-lane roundabouts reduced total crashes and property-damage-only crashes by 51% and 50%, respectively, while multilane roundabouts were associated with increases in total crashes and property-damage crashes. 

But it was the double-teardrop that outdid all the other roundabout versions, delivering an 84% reduction in injury crashes — and a 63% drop in total collisions. 

Experts note that roundabouts reduce crashes because the circular median and tight turning radius force drivers to slow down. The most severe types of intersection crashes — right-angle, left-turn and head-on collisions — are also unlikely because vehicles are no longer crossing perpendicularly.

Double-teardrop roundabouts work the same way, except the circle is squashed at the center. This design allows them to be used in locations where surface roads intersect a freeway or other high-speed thoroughfare.

According to IIHS researchers, that may explain why double-teardrop roundabouts showed such significant safety benefits in Carmel. They were installed at crossing points with higher-speed roads at intersections that had more injury crashes in the period before the conversion than other converted intersections. These intersections also may have benefited more from the speed reductions associated with roundabouts.