A Roundabout Highway Design

Indiana city takes control of state road, alters reconstruction project

Fast-track project turns highway into limited-access parkway, replacing six stoplight intersections with grade-separated roundabout interchanges

By Steve Kaelble

A renderiing of a finished overpass with roundabout. ( Renderings courtesy of American Structurepoint)

Sometimes, you just have to do the job yourself. That was the sentiment in the Indianapolis northern suburb of Carmel, Ind., when residents and city officials didn’t care for the upgrades planned for a section of state highway running through town. So they worked out a deal to take over the highway and implement their own plans to improve the thoroughfare.

The busy road in question used to be known as Indiana 431, or Keystone Ave. It is a north-south, divided highway that travels about four miles through Carmel, with a pair of lanes in each direction interrupted by six stoplights before linking with U.S. 31 near the north edge of the city. U.S. 31 is also a north-south, divided and signalized highway crossing Carmel, and the state’s plans to upgrade it into a limited-access freeway in a few years directly affected the future of Keystone Ave., which needed some updates of its own so it could better handle the extra traffic expected to divert off of U.S. 31 during that highway’s reconstruction.

On Keystone, “the Indiana Department of Transportation (InDOT) was planning to add a third lane and keep the traffic signals,” explains Jeremy Kashman, Keystone project manager for the city of Carmel. “When INDOT held meetings about adding the third lane, there were some outspoken people.”

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“The state needed to improve the corridor, but the city had its thoughts and the state had its thoughts,” says Craig Parks, project development director for Indianapolis-based architectural and civil engineering firm American Structurepoint. The city, he explains, already disliked how Keystone served as a barrier to east-west vehicle, bicycle and pedestrian traffic. Adding yet another lane in each direction seemed like it would just add to the east-west problem without achieving that much improvement on Keystone.

“The mayor talked to INDOT about relinquishing control of the road to the city of Carmel,” Kashman says. In September 2007, the state agreed, and handed the highway to the city along with $90 million to fix it up however the city saw fit. The only catch—upgrades had to be designed and completed before the U.S. 31 project was scheduled to begin.

Thus began a fast-track project to re-create the highway into Keystone Parkway, a limited-access thoroughfare with the six stoplight intersections replaced by grade-separated, roundabout interchanges. Those having trouble picturing that kind of an interchange can be forgiven, Parks notes: “These are the first in this state and I believe some of the first in the Midwest.”

Once the ball got rolling, American Structurepoint had no time to lose. “The state relinquished control of Keystone in late 2007,” Parks observes, and to make the completion deadline, “we had to have the first two intersections under construction in April 2008.”

Milestone Contractors won the bid to work on those two interchanges, at 106th and 126th streets. Later in 2008, Walsh Construction won the contract to build the 136th Street interchange. Next came a bid for the interchanges where 116th Street and Carmel Drive cross Keystone, which are near enough to each other that they’re being built as one combined exit from Keystone. Chosen for that part of the project was Rieth-Riley Construction Co. Finally, bids are being sought this summer for the last of the six interchanges, at 131st Street, also known as Carmel Drive.

Crews work on lowering the grade of Keystone Parkway. ( Photo courtesy of American Structurepoint)

The general plans for all interchanges includes lowering the grade of Keystone, raising the cross street, building a bridge across the highway and adding exit and entrance ramps. The need to add those ramps in a corridor with little right-of-way to spare was one of the primary reasons for choosing roundabout designs, Parks says. “If you look at a standard diamond interchange, you have to maintain a certain space between the ramps.” Even in the tightest diamond interchanges, the two intersections involving the ramps and the cross street on either side of the highway need to be some 300 to 400 ft apart, which means that the ramps have to flare out from the highway in order to add that distance.

“This corridor is residential for the most part, with one section in the middle being retail and commercial,” Kashman says. By building a signal-free roundabout system crossing the bridge at the top of the entrance and exit ramps, the designers could keep the ramps parallel and close to Keystone rather than flaring them out, minimizing the need to acquire more land.

American Structurepoint designed what have been called “teardrop” roundabouts for the interchanges—rather than one large traffic circle, which would have required a pair of bridges across the highway for each interchange, the design features two small roundabouts where the ramps on each side connect. The designs were blessed by a United Kingdom consulting firm brought in because of its extensive experience with European highway roundabouts.

The Keystone interchanges are quite compact—one of the facets that made the project interesting for Milestone Contractors, according to Mark Thompson, vice president and area manager. “This project has been a challenge,” he says. “It consists of many features in a relatively small geographic area, with bridges and many retaining walls and roadway underneath the bridges. We had to work with existing utility services and also under traffic, and under adverse weather conditions.”

And, of course, the company had to work quickly to meet the project’s compressed schedule. It was awarded the job in the spring of 2008, and by December already had all of the highway’s travel lanes at its two interchanges open at their new lower grade. By the first day of summer 2009, Milestone’s part of the project was virtually complete, thanks to meticulous planning and coordination.

Walsh Construction’s part of the project, which was started later, is about 50% complete, according to Kashman. The Rieth-Riley piece is the most complex part of the Keystone Parkway project, and is still in the early stages. Utility issues that required a couple of weekend closures of Keystone had to be tackled first. A 45-day closure in the area of the 136th Street work also was required to prepare the site. Kashman says being able to do so ultimately shaved two-thirds of a million dollars from the price tag.

To smooth out traffic flow on Keystone Parkway and simplify crossing it, the project is replacing common intersections, like the one shown at left, by lowering Keystone Parkway, raising the cross street, building a bridge across the highway, adding entrance and exit ramps, and using roundabouts to control traffic on the cross street. A completed interchange is shown in the rendering at right. ( Photo courtesy of American Structurepoint)

“We’ve had our share of challenges with the utilities,” he observes. In various places where the roadway was to be depressed, utility companies had to reroute linesto the north or the south, Parks adds.

Funding has been another challenge. Originally, the $90 million provided by the state was expected to cover all six interchanges. Later estimates put the total cost of the project as high as $149 million. The most recent estimate announced by the city pegs the total at $120 million to $125 million.

Kashman says the fluctuations have come as material prices and other costs have shifted wildly in the changing economic climate, and the total price has diminished as bids have come in below expectations. What’s clear now is that the original $90 million will cover five of the six interchanges, and the city is exploring options to pay for the final piece. “That is all being discussed in the City Council,” Kashman says. “They’re working on different ways to fund the project.”

While that crucial component remains a question mark, Carmel drivers are already providing answers about how well roundabout interchanges can function, now that the first two are open. “People are handling it really well, and the roundabouts are handling traffic very efficiently so far,” Kashman says. For example, the 126th Street interchange used to be marked by east-west backups of up to a quarter mile during rush hour, he says. Now, such jams are nearly non-existent, and Keystone traffic speeds by underneath uninterrupted.

There’s no place more fitting than Carmel for a set of roundabout highway interchanges. In recent years, the city has had a love affair with roundabouts, replacing roughly three dozen signalized or stop-sign intersections with the circular traffic handlers. Carmel, in fact, has become such a roundabout capital that a national roundabout convention will be held there in 2011.

Proponents like the safety of roundabouts. “We have seen a 78% reduction in personal-injury accidents at these intersections,” Kashman says of the city’s other roundabouts. What’s more, the average dollar value of the property damage incurred in Carmel’s roundabout crashes is about a quarter of the property damage sustained by victims of accidents in traditional signalized intersections, he says.

Roundabouts also tend to be more environmentally friendly, Parks says. “You have less power required for signals, and without the starting and stopping the emissions are lower.”