CTRE  en route
Winter 2006

CTRE en route is published twice a year to inform readers about CTRE's projects and programs.

Coralville, Iowa roundabout

The First Avenue and Holiday Road roundabout in Coralville. Photo by Scott Larson

Address: CTRE, 2901 S. Loop Drive, Suite 3100, Ames, IA 50010

Phone: 515-294-8103
FAX: 515-294-0467

Website: www.ctre.iastate.edu/

CTRE Communications:
Marcia Brink
CTRE en route editor:
Michele Regenold

CTRE is an Iowa State University center.

Iowa State University: Becoming the best

Roundabouts in rural America

Hillary Isebrands, an Iowa State University doctoral student in civil, construction, and environmental engineering and CTRE research assistant, is studying whether modern roundabouts on rural Midwest highways can reduce crash severity. She is also negotiating grants with the Iowa Department of Transportation, the Federal Highway Administration, the Minnesota Department of Transportation, and Minnesota's Local Road Research Board to help develop guidelines for considering and building roundabouts.

And she's working with Shauna Hallmark, an Iowa State associate professor of civil, construction and environmental engineering and CTRE transportation engineer, to assess whether roundabouts can reduce vehicle emissions by decreasing idling time and creating fewer stops at intersections.

Reasons to build roundabouts

Isebrands said there are three good reasons to build roundabouts:


Research shows roundabouts reduce crashes by 50 percent and reduce the severity of crashes by up to 80 percent, Isebrands said. That's because drivers have to reduce speeds to get around them, especially if a roundabout replaces a highway intersection controlled by two stop signs. Roundabouts also reduce potential crash points from 32 in a four-way intersection to eight. And roundabouts eliminate head-on crashes and make "T-bone" crashes (one car's front crashing into another car's side) much less likely.

Traffic flow

Traffic at busy intersections doesn't pile up behind roundabouts, Isebrands said. Drivers entering a roundabout don't have to stop, but they do have to yield to vehicles already in the circle. Cars continuously move in and out of roundabouts, increasing traffic flow and intersection capacity.

Another tool

Adding stop signs or traffic signals isn't always the solution to a problem intersection. Sometimes traffic signals are only warranted during peak travel times. And traffic signals can result in more severe crashes. Roundabouts give road designers and transportation engineers another solution to consider.

For all those reasons, roundabouts are on their way to Iowa's highways.

Roundabouts in Iowa

John Abrams, an engineer for the rural design section of the Iowa Department of Transportation, said the department will build its first highway roundabout this summer. That roundabout will be at the south intersection of U.S. highways 63 and 34 in Ottumwa.

The roundabout should improve a three-way "spaghetti jumble" at the intersection, he said.

And will Iowans see more highway roundabouts over the next few years?

"I would definitely say yes," Abrams said. "Once the people of Iowa notice this one, there will be more of a demand."

Coralville has built two roundabouts since 2002, has one under construction and another in the planning stage. Scott Larson, Coralville's assistant city engineer, said the city built the first two roundabouts as alternatives to adding turn lanes and traffic signals.

He said drivers seem to like them and there have been few negative comments.

"People tend to appreciate how a roundabout is an alternative to traffic signals," Larson said. "As people learn to drive roundabouts -- and there is a short learning period for some -- they begin to appreciate how they can help themselves get through the intersection more smoothly by yielding and anticipating gaps in traffic, instead of being at the mercy of a stop sign or red light."

But will roundabouts work out in the country? This is Iowa, after all. What happens when a tractor pulls a disk plow into one?

Isebrands, who worked six years as a highway designer in Wisconsin and returned to Ames in 2003 for graduate school, answered by clicking through her laptop computer. And there was a picture of a Kansas roundabout with room for three big trucks pulling three long trailers.

So there's room in a roundabout for tractors.

And Isebrands thinks there's room in Iowa for roundabouts and their safety features.

"I think there's a place for them," she said. "Roundabouts are a proven safety alternative for reducing crash severity at intersections and this puts Iowa one step closer to reducing the number of Iowans who die every year in crashes."

For more information

Contact Hillary Isebrands for more information about her research: hillaryi@iastate.edu or 515-294-7188.