State highway officials plan to begin building North Carolina's first "diverging diamond" interchange at Interstate 77's Cornelius exit by early 2012.
The interchange at Exit 28 would be only the second of its kind in the nation. The first has been alleviating traffic tie-ups in Springfield, Mo., since last summer.
Officials with the N.C. Department of Transportation in Raleigh announced plans for the interchange at a Cornelius Board of Commissioners meeting Monday.
The Cornelius board voted 5-0 to endorse the "diverging diamond" concept for Exit 28.
"Diverging diamond" interchanges are being planned in eight other states, from Oregon to New York, and eight or nine N.C. municipalities are also considering them, said John Conforti, who oversees N.C. DOT project development in Western North Carolina.
Conforti told the Cornelius board that Exit 28 is ripe for a "diverging diamond" based on its comparatively high rate of wrecks and chronic left-turn backups.
Conforti said the Springfield intersection has seen a 60percent reduction in wrecks since building its "diverging diamond" interchange. The interchanges are also pedestrian- and cyclist-friendly: Traffic signals stop vehicles to allow walkers and bicyclists to cross, he said.
A Missouri Department of Transportation official told the Observer earlier this year that the interchange has worked as planned, also alleviating bad traffic tie-ups.
Building such an interchange is far less expensive than constructing a more traditional one, Conforti said, and takes far less time to complete.
DOT figures a "diverging diamond" at Exit 28 would cost up to $5.7million, and probably only $5million, Conforti said. A more traditional interchange would cost about $34million, he said.
That's in part because DOT wouldn't have to purchase any right-of-way for the Exit 28 "diverging diamond" project, he said.
"We don't see any right-of-way acquisition costs at all," Conforti said.
If construction starts in early 2012, work could be complete before that year's end, he said.
The project would require expanding several hundred feet of the road on both sides of the interstate bridge, Cornelius project manager Doug Barrick has said, but the I-77 bridge deck wouldn't have to be rebuilt, thereby saving tens of millions of dollars.
Cornelius already has received a $6million federal earmark through U.S. Rep. Sue Myrick, R-N.C., toward improving the exit.
With a "diverging diamond" interchange, motorists on West Catawba and Catawba avenues wouldn't have to stop at a traffic light to turn left onto the interstate.
Northbound and southbound traffic exiting I-77 would flow more freely, no longer stacked up at lights where the exit ramps meet West Catawba-Catawba.
"Is there a negative here?" Cornelius commissioner Jim Bensman asked before the vote.
"At this point in time, no," Conforti replied. "And it looks like it's going to be fairly popular" nationwide.
The "diverging diamond" concept originated in France, Conforti said. And while only one U.S. intersection has one so far, he said, officials expect "a dramatic reduction in accidents" where they're built.
"We're all shocked an idea this good could come out of France," Cornelius commissioner Dave Gilroy quipped before the board's vote. "... Safe to say, our feelings (about "diverging diamonds") are extremely positive."'
A diagram of traffic flow at a "diverging diamond" interchange is available at www.435ddi.com, part of the Missouri Department of Transportation's Web site.