Local News

Innovative 'diamond' highway interchanges coming to NC

Posted September 24, 2013 5:05 p.m. EDT
Updated October 25, 2013 10:36 a.m. EDT

— Kansas City, Mo., has long been known for great blues music and barbecue. Now, it’s getting attention for one of the newer innovations in the traffic world.

A Diverging Diamond Interchange, or DDI, reduces congestion through intersections by allowing two directions of traffic to cross temporarily to the left side of the road, according to the state Department of Transportation. The movement provides easier access to an interstate, especially during peak traffic times.

“It looks confusing or intimidating because you criss-cross traffic,” explains Bastian Schroeder, a traffic engineer with North Carolina State University’s Institute for Transportation Research and Education.

Triangle drivers may soon experience the unusual interchange for themselves. There are five of them under construction across the state, and engineers are also considering building one in Raleigh, on South Saunders Street at Interstate 40, as part of the big 11.5-mile rebuild project.

Schroeder said the unconventional design keeps traffic flowing faster. Drivers trying to make a left onto a freeway ramp don't have to wait for a gap in oncoming traffic.

“The left-turning movement only crosses over the first cross-over, and then is free flowing onto the freeway,” Schroeder said.

Travel times at the Kansas City interchange dropped up to 70 percent, he said.

The Diverging Diamond is also a money-saver, requiring no extra lanes or traffic signals.

“They're insanely cheap (to build),” Schroeder said. “They're incredibly cheap compared to the alternatives.”

Traffic engineers at N.C. State have been monitoring DDIs around the country for several years as part of a federal safety study. The results will probably be released next year, but Schroeder says drivers generally like the design.

“Drivers are not sure how well they'll work, and then as soon as they open, they really like them,” he said.