Unusual design slashes injury crashes for Roundabout City

August 19, 2021

Double-teardrop roundabout in Carmel, IndianaDouble-teardrop roundabout in Carmel, Indiana(American Structurepoint, Inc. / Above All Photography)
Double-teardrop roundabout on Keystone Parkway in Carmel, Indiana
American Structurepoint, Inc. / Above All Photography

An unusual kind of roundabout has delivered big benefits for the Indianapolis suburb dubbed “Roundabout City,” a recent study from the Insurance Institute for Highway Safety found.

The double-teardrop design that Carmel, Indiana, installed along Keystone Parkway and at other busy intersections slashed injury crashes by 84 percent and reduced all crashes at these locations by nearly two-thirds, the IIHS analysis showed. Combined, the double-teardrop interchanges and more conventional roundabouts cut injury crashes in half.

“Our results show that Carmel’s double-teardrop roundabouts are remarkably effective in preventing the most severe crashes,” says IIHS Vice President of Research Jessica Cicchino. “The main reason is that their design allows them to be installed at the most dangerous intersections.”

Carmel boasts more roundabouts than any other city in the United States, with more than 100. Mayor James Brainard, who has made their construction a key focus of his seven consecutive terms, credits them with reducing emissions, easing traffic flow and keeping crash numbers low, even as the city’s population has soared from around 25,000 in the 1990s to nearly 100,000 people today.

To quantify how much they have affected road safety, IIHS researchers examined crash data from 64 roundabouts over the two years before and after their construction. Because information on historical traffic volumes was not available, they compared crash numbers from each converted intersection to a conventional intersection with similar characteristics nearby.

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 IIHS analysis showed that the conversion to roundabouts resulted in a 47 percent reduction in injury crashes overall, compared with the number the researchers projected would have occurred if roundabouts had not been adopted.

Single-lane roundabouts reduced total crashes and property-damage-only crashes by 51 percent and 50 percent, respectively, while multilane roundabouts were associated with increases in total crashes and property-damage crashes. Injury crashes dropped 50 percent at single-lane roundabouts and 15 percent at multilane roundabouts, though these estimates were not statistically significant.

The impact of the double-teardrop roundabouts was more striking. At locations where that design was used, injury crashes fell 84 percent, and total crashes dropped 63 percent, the researchers found.

Roundabouts reduce crashes — especially injury 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, also known as “dogbone” interchanges, work the same way, except the circle is squashed at the center (see diagram). This design allows them to be used in locations where surface roads intersect a freeway or other high-speed thoroughfare.

Double-teardrop roundabout

double teardrop roundabout

That may explain why double-teardrop roundabouts showed such large 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.

For example, Carmel converted the former State Road 431, a congested state highway, into the Keystone Parkway. Scrapping the Indiana Department of Transportation’s plan to widen the major arterial road from four to six lanes, Carmel instead replaced its major intersections with double-teardrop roundabouts that allow traffic on the parkway to flow much as it would on a true freeway.

For everyday users, the reduction in traffic congestion associated with all types of roundabouts is more noticeable than the drop in crashes. Depending on the intersection, previous research has shown roundabouts can reduce traffic delays by as much as 90 percent.

Not surprisingly, locals have embraced them — though construction-related traffic and the costs of building and beautifying the roundabouts can sometimes generate ire.

“Generally, people who don’t support them don’t live here, so they don’t have much experience with how well they work,” says 38-year-old Brandon Lust, a cycling and pedestrian advocate who recently moved to Carmel specifically because of its street design. “Driving in Carmel is unlike any other American city. I can drive from one end of Carmel to the other without ever going through an [ordinary] intersection.”

Heather Ward Miles, a 40-year-old artist, agrees.

“Now, every time I approach something that’s congested because there’s no roundabout, I feel irritated,” she says. That’s especially noticeable on the state highways near the Keystone Parkway that are still using conventional interchanges, she adds.

“[On Keystone Parkway] there are no backups,” she says. “On [Interstate] 465, when you’re trying to get off to other streets like [State Road] 37, there are big backups because people are stuck at lights.”

The increase in minor crashes at multilane roundabouts adds to a growing body of evidence showing that drivers find them especially challenging. An earlier IIHS study of two multilane roundabouts near Bellingham, Washington, for instance, found that a year after construction was completed, more than 40 percent of drivers  remained confused about what speed to drive and which lane has the right of way when exiting.

On the other hand, a larger study of roundabouts in Washington state found that the problem improves with time, as the number of crashes at two-lane roundabouts decreased on average 9 percent per year. Since the current study of Carmel’s roundabouts examined the number of crashes two years before and two years after each roundabout was built, that learning curve was still in its early stages.

The benefit from all three types of roundabouts in Carmel may actually have been even larger than estimated in the new study, says IIHS Research Transport Engineer Jin Wang, the lead author of the study. Carmel’s population nearly doubled between the beginning and end of the study period, and, according to city officials, roundabouts tend to be built in the highest-growth locations first.

“If traffic increased more at roundabouts than it did at other intersections, our estimates of the crash reductions at all roundabouts would be too low and our estimates of the increase in fender benders at multilane roundabouts would be too high,” Wang says.

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