It's called a "diverging diamond." It's one of the newest concepts in interchange design. And it could be introduced in Illinois for the first time at an often-clogged intersection in the suburbs -- I-88 and Route 59 in Naperville.
As yet, the diverging diamond concept isn't the most likely option for that intersection, but it hasn't been ruled out.
The design essentially reroutes drivers to the other side of the road atop overpasses in an effort to keep traffic flowing more freely and eliminate most left turns in front of oncoming traffic. Motorists cross back over to the regular side of the road on the other side of the overpass.
It's a road configuration that's gaining momentum among traffic experts, but has rarely been put into practice. Currently, there are only three diverging diamond interchanges in the U.S. - two in Springfield, Mo., and one in Salt Lake City, Utah. They've been in use in France since the 1970s.
The design is touted as being a safer alternative to traditional interchange designs and is cheaper in some cases.
Interchange improvements are part of the Illinois Department of Transportation's $86 million widening and redesign of Route 59 from Ferry Road to Aurora Avenue. However, state transportation officials said the current preferred option is improving on the traditional design there.
"We're going to be adding a new lane in each direction and storage lanes as well," said John Baczek, IDOT's project and environmental studies section chief. "Some of the preliminary information we've reviewed on diverging diamond interchanges suggests they may be more appropriate at places with less volume."
The interchange handles about 50,000 cars daily, state and local transportation officials said. During rush hours, it can take motorists a half-hour to access I-88 from less than a half mile away because of congestion.
The diverging diamond design along with additional lanes would significantly reduce congestion, said Bill Novack, a Naperville city engineer. "We ran a simulation on it with that design and it showed a 35 percent decrease in congestion," he said.
Donald Saiko, a project manager for the Missouri Department of Transportation, said the first diverging diamond interchange at I-44 and Route 13, which opened last year in Springfield, was a tough sell. However, when officials were able to show how the design worked, the public became supportive.
"The biggest hang-up is always crossing over to the other side of the road," Saiko said. "We had just a few problems at first, but we looked at crash data at that location for the first six months and it showed we had reduced crashes by 50 percent and we didn't have any of the high-severity crashes that cause injuries anymore because you don't have anyone turning left in front of oncoming traffic."
Baczek said safety and operational success are the top priorities for adjusting the Naperville interchange, but current congestion keeps high-velocity "T-bone" crashes to a minimum. However, some are concerned that adding extra lanes and reducing congestion at the interchange with the current design will make T-bone crashes at the interchange more common in the future.
Saiko also said the diverging diamond design reduced the cost of the interchange redesign in Missouri by two-thirds. Instead of spending $10 million on another design, the diverging diamond design cost $3.2 million, he said.
Baczek said while cost is a consideration for the interchange redesign in Naperville, there's likely not much difference between the preferred design and the diverging diamond. If the state had looked into creating a cloverleaf design at the interchange, that might have increased the cost. Baczek said the state didn't examine a cloverleaf design because it would have required buying much more land in the area.
Baczek also said the diverging diamond concept at the Naperville interchange is not dead, it just remains a second option. If officials reconsider the design, more public hearings would likely be held. Meanwhile, Baczek is hopeful the engineering and environmental studies for the Route 59 project will be approved this fall so land acquisition can begin and construction can get started in a year or two.