There has been a 60 percent reduction in accidents at the Catawba Avenue/I-77 Exit 28 interchange in Cornelius since the diverging diamond interchange (DDI) opened last year.
For town and state officials who, prior to construction, emphasized the better safety record of existing DDI intersections in other parts of the country, the new figures would appear to bear out their predictions.
Traffic analysts attribute the better DDI safety record primarily to the reduction in the number of traffic conflict points when compared to a traditional interchange design. In the DDI design, motorists cross the interstate on the left, rather than the right side of the road, and then turn left to enter the interstate.
“We looked at a nine-month period prior to construction beginning, and then a nine-month period after construction was completed,” said Assistant Town Manager Andrew Grant. “The results: pre-DDI: 30 crashes, post-DDI: 12 crashes. That’s a 60 percent reduction.”
Better safety records are not the only plus for the DDI design. They can also be constructed, as was done in Cornelius, using the existing bridge structure and the existing right-of-way, eliminating the cost of building new structures and purchasing additional right of way.
However, even with the enhanced safety statistics, the Exit 28 DDI is still prone to congestion because of the close proximity of the “bookend” intersections: U.S. 21 to the east; Torrence Chapel Road to the west. “We’ve known for years that in order to improve the traffic flow in the entire area, improvements would be needed at our bookend intersections, too,” Grant said. “The close proximity of the ‘bookends’ to the interchange and the large volume of traffic in the area during all times of the day causes periodic backups into the DDI itself.”
Design work is underway to improve these “bookend” intersections with Catawba Avenue, and officials are confident that the intersection will be even more efficient when that work is completed.
Meanwhile, the DOT is planning to construct additional DDI interchanges throughout the state. “A DDI conversion is planned at the I-77/Gilead Road (Exit 23) interchange in Huntersville,” said DOT spokeswoman Jordan-Ashley Baker. “In fact, it is currently scheduled to be let to contract in December 2017.”
The Gilead Road interchange, which is also where I-77 narrows to two lanes for northbound motorists, is prone to extensive delaysbecause of the limited number of lanes available to motorists, plus difficult left turn movements facing oncoming traffic.
Another DDI is planned for Statesville in conjunction with the improvements underway at the I-40/I-77 interchange. This DDI will actually be at the interchange at I-40/U.S. 21, which is west of where the interstates intersect and will also include work on U.S. 21 from south of I-40 at BB&T to north of Glenway Drive.
Work on this intersection should be completed by early 2017.
Baker said the first DDI in the state was built at the intersection of Poplar Tent Road and I-85. Another one is now open at N.C. 73 and I-85; a third DDI opened a few months ago at Mallard Creek Road and I-485 when the final leg of the Charlotte beltway was opened. DDIs will also be opening soon in Kernersville, Lumberton and Ashville.
Nationwide, DDIs are catching on quickly. Construction crews just completed the first one in the Chicago area, near Naperville, Ill., on Intersate 88. New York State is joining the ranks of DDI supporters, getting set to build their first DDI on Interstate 590 in Brighton, N.Y., near Rochester.
Dave Vieser is a freelance writer. Email firstname.lastname@example.org.