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Status Report, Vol. 45, No. 11 | November 3, 2010 Subscribe

Indiana city takes roundabout path to safer roads

Main Street and 4th Avenue in Old Town Carmel, Ind.

Carmel, Ind., is a rapidly growing, prosperous community outside Indianapolis known for good schools, abundant shopping, and a vibrant arts scene. But one basic element of urban infrastructure is scarce in the city: traffic lights.

This is no accident. If Mayor James Brainard could rid Carmel completely of the darned things, he probably would. His preferred type of intersection is a roundabout, and he says Carmel has more of them than any other U.S. city.

Brainard credits roundabouts with keeping the number of traffic injuries from growing along with the city's road network. In 2003, there were 252 crashes causing injury on 220 miles of roads, according to Carmel officials. By 2008, the city had 395 road miles, but injury crashes went down slightly to 223. More than 2 dozen roundabouts opened in Carmel in the intervening years. By the end of 2010, the number of city-built roundabouts is expected to reach 55, including 6 roundabout-style interchanges just completed in September along the busy Keystone Parkway. Approximately a dozen more have been built by developers of residential neighborhoods. In contrast, traffic lights number a mere 41.

Carmel will get a chance to show off its roundabouts in May, when it hosts the Transportation Research Board's International Roundabout Conference.

"There are a huge number of roundabouts to see, and they come in different configurations," says the research board's transportation safety coordinator, Richard F. Pain, explaining the group's choice of Carmel to host the conference. "They're very, very clever in some of the designs. They take the concept of the roundabout, and they make it fit."

Roundabouts, which were developed in the 1960s in the United Kingdom and are common in much of Europe, used to be rare in the United States. However, the advantages they hold in terms of safety, congestion mitigation, monetary savings, and aesthetics are making them increasingly popular here. Much smaller than traffic circles, or rotaries, they force drivers to slow down to negotiate tight curves. Vehicles entering a roundabout are required to yield to traffic already in the circle (see "When roadway design options are wide open, why not build a roundabout?" Nov. 19, 2005).

Roundabouts essentially eliminate the potential for the most dangerous types of crashes — right-angle, left-turn, and head-on collisions — because traffic moves in a single direction. Compared with traffic signals, they also reduce the likelihood of rear-end crashes because no one speeds up to make a yellow or green light or abruptly stops because a signal turned red.

The crashes that do occur at roundabouts generally are not severe because vehicles move more slowly than they do at conventional intersections. A 2001 Institute study of 23 intersections in the United States found that converting intersections from traffic signals or stop signs to roundabouts reduced injury crashes by 80 percent and all crashes by 40 percent.

Beginning in the 1990s, Carmel's rapid population growth — from 25,000 in 1990 to about 70,000 today — forced the city to convert many of its 4-way stops to something better at managing high traffic volumes. The obvious answer would have been stop lights, but Brainard, who had spent time in England years before, asked the city's engineering consultants to design a roundabout.

"They didn't want to do that because they were confusing modern roundabouts with the old rotaries," which are generally considered confusing and don't have the same safety benefits, the mayor recalls. So he went to Purdue University's engineering library and brought back journal articles on roundabouts to the engineers. They agreed to give it a try, and Carmel's first roundabout opened in 1997.

Carmel has learned a lot since those early days. Back then, the U.S. Department of Transportation didn't have any specifications for roundabouts.

"We used the Australian roundabout specifications, just flipping them over," since Australians drive on the left side of the road, Brainard recalls. "It really wasn't what we needed."

This year, 2 of Carmel's early roundabouts were rebuilt according to the city's current standards. Drivers now enter the roundabouts at sharper angles, forcing them to slow even more. The slower speeds are not only safer, they also create bigger gaps between vehicles for other vehicles to enter, thus improving the traffic flow.

Brainard says that in the city's experience roundabouts are invariably cheaper to build than intersections with signals. There's no initial purchase of signal equipment and no need to inspect and calibrate it as the years go by.

"We've landscaped the middle of our roundabouts in most cases, and it helps property values in the area," Brainard adds. "It's better to have a beautiful flower urn out there in the middle of a circle than blinking lights outside your bedroom window."

If there was skepticism on the part of drivers at first, Carmel's residents today are largely pleased with their roundabouts, Brainard says.

"Change is scary to people," says Mo Merhoff, president of the Carmel Chamber of Commerce. But roundabouts have won support by reducing travel times during rush hour, she says. "I'm one of those people who actually plan my route based on where I can utilize roundabouts."

Keystone Parkway project

Keystone project melds two traffic engineering strategies

Carmel's success with roundabouts emboldened the city to take on the ambitious Keystone Parkway project. Unsatisfied with the state's plan to add lanes to ease roadway congestion, Carmel reached an agreement with the Indiana Department of Transportation to assume ownership of the road. The city's modifications involved lowering Keystone and building roundabout interchanges connecting the exit lanes and the cross streets. The interchanges are shaped like double teardrops (above) rather than circles. City officials believe the grade separation combined with the roundabout interchanges will improve safety and shorten commute times.

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