Here’s how a diverging diamond interchange works
Engineers for the state Department of Transportation think the strategy for fixing two of the worst highway interchanges in the Triangle should involve driving on the wrong side of the road.
The NCDOT has proposed building what are known as “diverging diamond” interchanges in two places in the Triangle: At the Beltline and Western Boulevard in Raleigh and at Interstate 40 and N.C. 42 in Johnston County. The scheme involves crisscrossing traffic at either end of the bridge over the highway in a way that eliminates left turns and the amount of time drivers spend sitting at red lights.
The diverging diamonds would be the first in the region, though not the first in North Carolina. There are four in the Charlotte area and one each in Asheville, Greensboro, Lumberton and Wilmington.
On paper, the diamonds look daunting. At recent public meetings about the two highway projects, people who encountered the unconventional designs for the first time often stared at them for a while, trying to get their head around how they work.
“I don’t have a good feel for it yet,” said Gina Miller, a real estate broker studying a diagram of the proposed diamond at N.C. 42, where she owns and markets property. “I’m still trying to figure it out.”
NCDOT had hung two large maps of the planned N.C. 42 interchange on the wall of Garner United Methodist Church on Monday evening. The remaking of the N.C. 42 interchange will be done in conjunction with the widening of an 11-mile stretch of I-40 from Raleigh into Johnston County starting in late 2018. In between the maps, an NCDOT video showing how traffic moves through a generic diverging diamond was projected on a screen on a continuous loop.
Bruce Wilkie, retired from Nationwide Insurance, has lived about 2 miles east of N.C. 42 for 17 years and crosses the bridge just about every day to get to his gym, his bank and the Food Lion. He thinks the new design will be a big improvement.
“I think the biggest challenge will be people getting used to it,” Wilkie said. “Just like roundabouts.”
Diamonds hailed as innovative
The diverging diamond appears to have been invented in the early 1970s in France, where only three were completed. The first one in the United States was built by the Missouri Department of Transportation at a busy interchange in Springfield in 2009. Popular Science magazine named it one of the top 100 innovations of that year. “The new design does away with risky left turns,” the magazine wrote. “The street approaching the highway now diverts to the left, and cars get uninterrupted access to the highway, which, experts say, can reduce clogging by as much as 60 percent.”
There are now 96 diverging diamonds in the United States, with another 20 under construction, including one on Interstate 85 in Concord, according to the Institute for Transportation Research and Education at N.C. State University.
There’s little dispute that the interchanges that NCDOT has chosen for the diverging diamonds in the Triangle need fixing. N.C. 42 is a conventional design that has been overwhelmed by growth, both with businesses clustering around the interchange and homes in nearby subdivisions. Western Boulevard, by contrast, is an outdated design with short ramps, tight cloverleafs that force entering and exiting traffic to crisscross and an on-ramp that merges onto the highway from the left.
Brian Brown, a GoRaleigh bus driver who lives just off the Western Boulevard exit from the Beltline, said the diverging diamond looked smart to him when he saw it for the first time at an NCDOT meeting in August.
“I love this. You don’t have to deal with weaving at the cloverleafs,” Brown said. “It’s a lot safer.”
Why diverging diamonds were chosen
NCDOT had plenty of people on hand at public meetings for both projects to explain how the diverging diamonds work. Among them was engineer Dean Sarvis of Stantec, the firm that helped choose the diverging diamond for the N.C. 42 interchange. Sarvis said other designs would either continue to be clogged with traffic or would require much more real estate to pull off. He says while they might look a little scary on paper, the interchanges are actually intuitive.
“As they drive through it, most drivers don’t know they’re on the quote-unquote wrong side of the road,” he said.
B.J. Johns, who lives in the Cleveland area of Johnston County near the N.C. 42 exit, watched the NCDOT video at this week’s meeting with his wife Martha and explained the virtues of the diamond to her. Johns said he had been through one out west somewhere and that it works if you simply follow the signs and the road.
“It’s a lot easier once you’re on it,” he said. “I was on one and didn’t even realize it until I was halfway through it. It was easy to get through with my truck and my camper.”
It will still be a few years before Triangle drivers actually drive on the diverging diamonds; construction on both projects is still a year or more from getting started and will take about four years to complete.
One advantage NCDOT has in winning people over to the diverging diamond concept is that the interchanges it has chosen for the concept are so awful now. Builder Wayne Baker is developing The Tapestry, a neighborhood of ranch-style villa homes for older adults, on Old Drug Store Road less than a mile from the interchange and lives about two miles on the other side of it. He echoes many others when he says the project is long overdue.
“It was a poor design originally, but development continued, and they were stuck with the original design,” Baker said. “I think everyone will be very optimistic to see changes coming.”