Skip to main content

Could a Diverging Diamond Interchange be coming to Southeastern North Carolina?

It's not every day that a driver gets to legally drive on the "wrong side of the road".  But if a recent NCDOT proposal for the US 74/76 interchange with NC 133 in Leland is approved, Brunswick County motorists will be able to do just that.

The DOT is considering building a "Diverging Diamond Interchange", also known as a DDI, as a replacement of the current and heavily congested diamond interchange when the US 74/76 causeway is widened in 2013.  The DDI is one of a few possible plans for the interchange when the $19 million project begins.

A diverging diamond is unique as the travel lanes on the surface road- in this case NC 133 - are flipped to the opposite side before and after the interchange.  This flip is controlled by a traffic light at each end.  Traffic to and from the interchange ramps also intersect at the signals. 

The result is left turns onto and off of the interchange ramps now go unimpeded - that is the driver does not have to worry about crossing traffic in the opposite direction when making the turn.  Also, the only traffic signals needed are the two at the location of the 'flips'.

In 2009, Missouri was the first state to install the 'diverging diamond'at MO 13 and Interstate 44 in Springfield.  A photo of the completed project is below. (Source: Missouri Department of Transportation)

Missouri installed this interchange as it saved nearly $4 million compared to building a higher powered traditional diamond with additional turning lanes.  With NCDOT putting together every possible penny that they can find, the DDI will be a very high possibility.  Missouri also boasts a lower accident rate at the MO 13/I-44 interchange, a reduction of 50 percent.

In addition to cost advantages, the DDI allows for interchanges to remain below capacity longer.  They are maximized if there is a high volume of left turns on a lower speed surface road with space limitations. 

There are drawbacks, however.  On the surface route, both sets of signals cannot be green at the same time for both directions.  The flow of traffic will be more controlled and through traffic - in this case on NC 133 - will have to come to at least one stop when going through the interchange.  And of course the obvious concern - motorists who would be confused and unfamiliar with the new traffic pattern.

Since construction of the diverging diamond in Springfield in 2009, a handful of DDI's have been constructed throughout the country.  If this DDI is built in Leland, it will be North Carolina's second constructed DDI.  The first, located on I-77 at Exit 28 in Mecklenburg County, is scheduled to open in 2012.

Story Link:
New interchange design considered on NC 133 in Leland ---Wilmington Star-News

(HT: Joe Babyak)


James Dunlop said…
The one in Leland would be anywhere between the first and seventh DDI in the state. The Cornelius one looks like it'll be sometime in 2013 instead of 2012, and there's three projects in Concord, two on 85 and one on the last new section of 485. These three are actually let to contract, although the Design-Build schedule puts them in late 2013/early 2014.

Another one in Kernersville on I-40 and one in Lumberton on I-95 are being worked on, however their schedule will most likely be behind the others. Right now, any one of these could be the first one completed. I'm betting on the two on I-85 in Concord, based on the construction schedules I've seen. Cornelius could slip in ahead of those, though.

Jim Dunlop
Anonymous said…
MODOT just opened another DDI, I-270 at Dorsett Rd, in St. Louis County, last week.
hotrod6147 said…
Was the new bridge at Old Kingsdale Road completed? Was this simply a bridge, or an interchange?

Popular posts from this blog

Old US 101; the San Juan Grade

While researching maps for California State Route 183 I noticed something interesting on the 1935 County Highway maps for San Benito and Monterey County.  From what it appeared it seems that there used to be a state highway running from US 101 south on San Juan Highway, through San Juan Bautista, south over the San Juan Grade to Salinas.  It turns out what I discovered was an a very old alignment of US 101 which was replaced by 1932.

The information relevant to the history of US 101 over the San Juan Grade is as follows:

-  The San Juan Grade was built in 1915 which presumably replaced Old Stage Road from Salinas to San Juan Bautista.  Presumably this was part of alignment adopted as Legislative Route 2 from San Francisco south to San Diego in 1909.  This history can be seen on 1931 edition of the California Highways and Public Works Journal and on

1931 Highways and Public Works Journal

CAhighways on LRN 2

-  By 1926 the San Juan Grade became part of US 101.  The San Jua…

The Tioga Pass Road

Last Summer the Tioga Pass Road over the Sierras in Yosemite National Park opened late due to the heavy snow pack during the previous winter.  Approaching the start of July the Park Service finally had cleared Tioga Pass, I headed up shortly after the 4th of July holiday during a lull in the tourist season.

The Tioga Pass Road runs from the Big Oak Flat Road east to US Route 395.  The Tioga Pass road is largely within the boundary of Yosemite National Park but is also partially on California State Route 120 east of the Tioga Pass entry station to US 395.  The Park Service maintained portion of the Tioga Pass Road serve as a implied connection between the two segments of CA 120.  The Tioga Pass Road is the highest road mountain pass in California with Tioga Pass which lies at 9,945 feet above sea level.

The Tioga Pass Road is very old with the eastern section up Lee Vining Canyon to the Tioga Mine being built in 1883.  The connecting section of the Tioga Pass Road from Big Oak Flat R…

California State Route 49, The Golden Chain Highway (CA 16 north over Yuba Pass to CA 89)

After completing California State Route 124 I took CA 16 to the eastern terminus to start my first Trans-Sierra route; California State Route 49/Golden Chain Highway over Yuba Pass.

As stated I joined CA 49 from the eastern terminus of CA 16 in Amador County.  CA 49 actually begins in Madera County to the south in Oakhurst at CA 41.  CA 49 is about 295 miles long and travels most of the traditional 1849 Gold Rush Country north from Oakhurst to CA 70.  If you want history and old towns then CA 49 is one of the best routes on the West Coast to see both.

To the north of CA 16 the next major junction is Signed County Route E16 in Plymouth which is on Shenandoah Road.  E16 is a 33.2 mile route which travels northeast to US 50. 

Plymouth dates back to the 1850s and is mostly known for a winery that dates back to 1856.  These photos are from Main Street looking west.

CA 49 generally is very rural and doesn't deviate much from when it was first signed back in 1934.  While CA 49 isn'…