Local News

Innovative 'diamond' highway interchanges coming to NC

Posted September 24, 2013
Updated October 25, 2013

— 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.  Diverging diamond interchanges coming to NC

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.

86 Comments

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  • Pseudonym Sep 25, 6:30 p.m.

    Quote from alana2: "I still have problems with the interchange at the Streets at Southpoint."

    No offense, but perhaps you should consider turning in your license if you have trouble navigating a simple SPUI.

  • Pseudonym Sep 25, 6:24 p.m.

    Quote from wildervb: "...Clover Leaf interchanges are the best..."

    HAHAHAHAHAHAHAHAHAHAHA!!!!!! If you honestly think that cloverleaves are the best then travel through the I-40/US-1/I-440/US-64 interchange in Cary between 7 AM and 9 AM, or between 4 PM and 6:30 PM. Make sure your affairs are in order and that your collision coverage is up to date.

  • Pseudonym Sep 25, 6:20 p.m.

    I drove through one of these things in Atlanta (Pleasant Hill Rd at I-85.) Honestly, if you can't figure one of these out, you've got problems, and you should do society a favor and turn in your license because you are intellectually unfit to be on the highways with the rest of us. These are EASY to navigate, and if they truly improve traffic flow, then more power to DOT!

  • RandolphBloke Sep 25, 3:53 p.m.

    These actually lower congestion quite a bit even though it wouldn't seem to be the case having lights. I think even bad Triangle drivers can adjust to these quickly. (I won't back that 100% because roundabouts proved to be too much for us.)

    There's also a modified version of this with very small underpasses that does away with the need for lights. On highly busy and congested areas those can drop traffic jams even more than this proposed type.

  • btneast Sep 25, 3:12 p.m.

    there will be no dedicated turn lane on the freeway where entering and exiting traffic compete for access (while entrants try to speed up and people exiting try to slow down)...

    especially trying to exit the highway in the same lane where traffic is entering the highway.

    The above statements are why cloverleafs are not ideal....

  • buchkshot Sep 25, 3:00 p.m.

    For an animated visual example of a DDI, go to youtube. There are about 10 examples of same.

  • anonymous99 Sep 25, 2:14 p.m.

    "This insanity means that one direction on a main thoroughfare will always have a red light."

    But it also means there will be no dedicated turn lane on the freeway where entering and exiting traffic compete for access (while entrants try to speed up and people exiting try to slow down)... And it means less space and materials because there are no clover ramps.

    It really makes a lot of sense if you think about it. A clover design does prevent left turns, but at the cost of a lot more infrastructure (four entry/exit points on each side of the highway instead of just two in this design).

    WRT the people saying "everyone stinks at driving", you're right. Drivers scare me, too - especially trying to exit the highway in the same lane where traffic is entering the highway. This design prevents that situation.

  • hp277 Sep 25, 1:16 p.m.

    I'm not sure I get it. If you want to allow traffic on one road to easily go onto another road, then why not use right turn lanes that then circle around to the road below. We have those everywhere. Having this diamond means there must be a stop light, whereas to two right-turns (one for each direction of the other road) requires no stop lights.
    paulej

    Exactly. A diamond/cloverleaf intersection, in use since the first interstates in the 50's, works just fine with all right turns and no stoplights at all.

    This insanity means that one direction on a main thoroughfare will always have a red light.

  • disgusted2010 Sep 25, 12:16 p.m.

    More of Tata's insanity. One day the people of NC will learn just how bad things are in DOT and rue the day that this carpetbagging bully was brought to NC by Art Pope.

  • anonymous99 Sep 25, 12:14 p.m.

    paulej - I think it's advantageous because it has on and off ramps for the freeway and the road that crosses it, allowing access to each direction from each direction with only one stoplight.

    Country Girlz Have MORE fun - Have you ever considered reading an article before commenting on it?

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