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MOORESVILLE, N.C. -- The state plans to build a second Interstate 77 diverging diamond interchange to relieve traffic snarls on one of the Lake Norman area's most congested roadways, at long-suffering Exit 36.

The new interchange would cut notoriously bad traffic backups on N.C. 150 in half, a study by the N.C. Department of Transportation found.

Mooresville's Exit 36 diamond is expected to cost state taxpayers about $8 million, with no local match required, Mooresville transportation planner Neil Burke said Friday.

Diverging diamonds allow two directions of traffic to temporarily cross to the left side of the road.

They are a French import, first used in the city of Versailles outside Paris in the 1970s.

Diverging diamonds also take less time to build and save tens of millions of dollars over a traditional interchange.

The Mooresville Board of Commissioners and the Iredell Board of County Commissioners will consider a resolution next week supporting the project.

DOT division engineer Mike Holder said the project still needs environmental and engineering studies and public hearings, so it's at least two or three years away.

But planning is under way, with a model of an Exit 36 diverging diamond already made, he said.

N.C. 150 is Mooresville's main east-west artery, one of the only highways that crosses Lake Norman, and is a conduit between Lincolnton and Salisbury.

The Exit 36 interchange is likely the town's most congested spot, with N.C. 150 averaging about 35,000 vehicles a day near I-77, Burke said.

A diverging diamond interchange would reduce congestion by allowing the through traffic on N.C. 150 to keep moving through the interchange area with minimal delay, he said.

In a diverging diamond, drivers proceed through a traffic signal at the entrance to the interchange and simply follow their lane to the opposite side.

Motorists needing to access the interstate turn left onto the on-ramp without having to stop for more traffic signals or wait for oncoming traffic to pass, N.C. highway officials said.

Markings and signals lead the way.

Safety improves because drivers make free-flowing turns entering and exiting the interstate, eliminating left turns against oncoming traffic.

Diverging diamonds provide better sight distance at turns, which leads to fewer crashes.

Switching to drive on the opposite side of the road might seem confusing at first. Also, there are no national statistics yet on accidents in diverging diamonds because they are so new.

But according to Time magazine, traffic accidents fell 50 percent during the first six months at the nation's first diverging diamond interchange, in Springfield, Mo., in 2009.

The Mooresville project isn't related to ongoing work to add an extra I-77 lane in each direction from Exit 36 south to Exit 35 that's under construction at Brawley School Road, Burke said.

That work is scheduled to finish by summer 2013.

A diverging diamond also is planned for I-77 Exit 28 in Cornelius, with construction scheduled for August 2013, DOT spokesperson Jennifer Thompson said.

As part of interstate construction in Concord, diverging diamonds are planned at I-485 and Mallard Creek Road and on Interstate 85 at both Poplar Tent Road and N.C. 73.

Charlotte-area diverging diamonds

  • I-85 at Poplar Tent Road and N.C. 73 in Cabarrus County. Land clearing and utility relocations are under way at both interchanges, which could open in summer 2013.
  • I-485 at Mallard Creek Road in northern Mecklenburg County. Should open in November 2014.
  • I-77 Cornelius Exit 28. Construction is scheduled for August 2013.
  • I-77 Mooresville Exit 36. No timetable has been set, but could be at least two or three years.

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