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Michelle Davies | The Journal Gazette
The Dupont Road-Interstate 69 interchange is one of the most congested in the region with left-turning traffic causing delays.

Project a diamond in the rough

Solution to Dupont/I-69 congestion entails travel on left on 2-lane road

– There are not many places in America where you can safely and legally drive on the left side of the road on a two-way street.

But that will soon be the case on Dupont Road where it crosses Interstate 69.

Even stranger, traffic engineers say the move will not only reduce congestion and delays in the area, but most drivers won’t even notice they’re on the “wrong” side of the road.

“The biggest response is that people say, ‘Wow, I just went through it and didn’t realize it,’ ” said Susan Doell, an Indiana Department of Transportation engineer and the project manager. “Especially for ours, with a pedestrian walkway in the middle, it’s not going to feel like you’re on the wrong side of the road.”

The project, which will begin in April as soon as the weather warms up, will transform the Dupont interchange from a traditional diamond interchange to a diverging diamond interchange, the first in the state. INDOT plans another diverging diamond for Interstate 65 at Greenwood, but that project is on hold until 2015.

A diverging diamond uses a traffic signal at each end to allow vehicles to cross over from the right side of the road to the left. That makes left turns – which usually require waiting for cross-traffic to clear – as easy as a traditional right turn.

“There’s no opposing traffic for the left turners,” Doell said. He added that the design not only speeds vehicles on their way, but the reduced number of points where there is opposing traffic makes diverging diamonds much safer and the site of fewer accidents.

The interchange is one of the most congested in the region, with long delays caused by traffic trying to turn left. Delays get so bad that cars waiting to turn left onto southbound I-69 sometimes block traffic trying to turn left from northbound I-69 onto westbound Dupont. That can cause traffic to back up on the off ramp, which is dangerous.

“What you find is people getting off I-69, if they want to turn left, they have to wait for the people turning left onto I-69,” Doell said. “Those lines are interfering with one another.”

But with the new design, every movement onto or off the highway is as easy as making a right turn.

In fact, the diverging diamond is so efficient in moving traffic that Doell said the design is expected to handle the traffic projected through 2030. The current level of service for the interchange is considered “failing.”

‘Great savings’

The design is efficient in another way, too: Its use of space.

INDOT considered four options for the interchange – doing nothing, building roundabouts, building a single-point interchange and building the diverging diamond.

Doing nothing was rejected because the interchange is already congested and causing delays. Roundabouts, like those used at the Union Chapel Road interchange, were rejected because they would require too much space.

Likewise, a single-point interchange, which uses one massive intersection at the center of the overpass, was also rejected for its space-hogging design.

The diverging diamond, however, will not require the purchase of any new right-of-way.

In addition to having to buy more land for roundabouts or a single-point interchange – which would be even more expensive given that the area is heavily commercial and land would be expensive – the bridge would have to be rebuilt to be much wider, raising the cost even further.

The cost for the diverging diamond, however – much of which will be paid for with federal highway money – is $3.5 million. The Union Chapel interchange, by contrast, cost almost $14 million, though it is substantially smaller.

“We realized a great savings there just by using the width of the bridge we have,” Doell said.

And the new interchange isn’t being built to cater only to vehicles: A new, multiuse trail will connect trails on either side of the bridge while giving pedestrians a safer crossing. The trail will be surrounded by decorative railings.

Weekend headaches

Of course, in an area that’s already congested, a major construction project will only add to the headaches.

Fortunately, officials say, while the work will be disruptive, it will require only two closures of the bridge, and those will be on the weekends.

INDOT spokeswoman Toni Mayo said the contractor, Brooks Construction, is working closely with the state and the two hospitals in the area to coordinate the temporary closures, which have not yet been scheduled.

The ramps for the highway will still be in use, but drivers won’t be able to cross the bridge. The detour will send traffic to Diebold, Union Chapel and Auburn roads.

Doell said the closures are required for crews to place the islands that will channel traffic over to the left side. “I sympathize with the people who have to go through there every day, but they’re going to be very happy when it’s done,” Doell said.

dstockman@jg.net

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