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NCDOT Creating New Patterns At Busy Interchanges

The Diverging Diamond Interchange.
Flickr Creative Commons

The state Department of Transportation is working on unusual interchanges designed to relieve congestion at high-traffic areas across the state. 

Crews are building seven intersections known as Diverging Diamond Interchanges.  They temporarily move traffic to the left side of the road before dumping them onto the freeway or shifting back to the right to cross the intersection. 

DOT engineer James Dunlap says it makes getting on and off the freeway easier.

"It's a free-flow left, so once you get past the first signal, you're not stopping again," Dunlap explains.

"At a traditional diamond, you'd stop at the first ramp, then you'd get in the left turn lane and have to wait for that phase in the signal, so you're waiting twice."

Dunlap says the DOT is considering a diverging diamond for South Saunders Street at I-40 in Raleigh.  He says the new intersections only make sense at spots that get a high amount of traffic, especially during rush hour. 

"It's not right for everywhere.  If it's low traffic on the ramps, there are going to be times of the day where you'll be sitting, waiting for the signal where with a traditional diamond, you probably would have been able to go.  It's a balancing act of looking at the peak periods of time, and then looking at the off-peak," says Dunlap.

They have cut down on accidents in other parts of the country, and Dunlap says they are cost effective because they don't need extra lanes or signals.

Will Michaels is WUNC's Weekend Host and Reporter.
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