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DOT previews interchange design for N.C. 280

Drawing shows how traffic would move freely through interchange while opposing traffic stops. Drawing shows how traffic would move freely through interchange while opposing traffic stops.

It may take some getting used to but state transportation engineers insist a new diverging diamond interchange at N.C. 280 and I-26 will make believers out of the traveling public.

The interchange, which was first used in France and is making its way gradually into use here, allows freer movement of traffic, is more efficient and costs less than upgrading traditional diamond interchanges.
Engineers with the state Department of Transportation previewed the proposed bridge design Monday during a public presentation. Maps and an animated display showed how traffic would move.
"Based on the analysis of this design compared to other designs, this is the most efficient for traffic movement, it operates with improved safety and at a reduced cost," said DOT engineer Michael Reese, a specialist in congestion management.

DOTFireChiefAsheville Fire Battalion Chief Dan Flynn shows DOT engineer Undrea Major a fire truck access issue. There's no denying that congestion plagues the Airport Road area. With the traffic volumes dramatically up because of new retail and the Asheville Regional Airport on the other side, the interchange is due for an upgrade to better handle the traffic load.

Unlike traditional diamond interchanges that require left turns across oncoming lanes, "motorists needing to access the interstate turn left on the on-ramp without having to stop for additional traffic signals or wait for oncoming traffic to pass," the DOT said in an information sheet.

Pedestrians use signalized crosswalks to reach a barrier protected pedestrian island in the middle of the road.

The N.C. 280 interchange is one of seven across North Carolina that are either under way or on the books. DOT transportation improvement plans show the interchange upgrade starting in October 2013 and costing $6.4 million. The DOT will finalize its decision on whether to go with the new design after public hearings.
"That decision hasn't formally been made yet but just looking at preliminary studies it's a really strong candidate," said project manager Undrea Major. Increasing capacity in a traditional diamond interchange would require widening the bridge from six to seven lanes, he said, dramatically raising the cost. The bridge, built in 1991, has an efficient rating of 82, which is high.
"I can tell you based on preliminary studies and traffic analysis, DOT's recommendation is going to go with the diverging diamond interchange," he said.

The first diverging diamond interchange, or DDI, in the U.S. was built on I-44 in Springfield, Mo. The Missouri DOT was able to upfit the interchange in six months while maintaining traffic during construction. The NCDOT has DDIs under construction at I-485 and Mallard Creek Road in Concord and on I-85 at Poplar Tent Road and N.C. 73. Besides Airport Road here, they're under consideration at interchanges on I-77 in Cornelius, I-40 in Kernersville, I-95 in Lumberton and U.S. 17 in Leland outside of Wilmington.

"This design is safer because there are fewer conflict points and it allows free-flowing movement," Major said. "But from a familiarity, this is nontraditional. There will be a learning curve people will have to go through. We won't become fully acclimated until it's on the ground."
Asheville Fire Department Battalion Chief Dan Flynn showed up at the public input session to check on access from Aviation Drive. The fire department expects to move into the airport fire department's station when the airport firefighters move into their new headquarters.
"We would need to be able to make this left" onto N.C. 280, Flynn said, pointing to the intersection just south of the interchange. It was unclear whether a concrete barrier shown on drawings would allow that turn. Major told Flynn that engineers would look at the options. Major said an emergency light that fire trucks could trigger would be a workable option.
Aside from that issue, Flynn said the design "looks pretty awesome." And it's needed. Because of congestion, "I just avoid the area completely."