HAYMARKET, Va. — When you first approach this bridge over Interstate 66 in northern Virginia, it may feel like you're driving on the wrong side of the road.

Because, in a way, you are.

"There were a lot of people who looked at me like I was a little nuts," says traffic engineer Gilbert Chlewicki, the designer who inspired this unconventional interchange. "Like, why are you putting me on the other side of the road?"

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NPR

Chlewicki agreed to meet at this intersection 35 miles west of Washington, D.C. to explain the workings of the diverging diamond interchange, as it's known. He was easy to spot, wearing a neon yellow vest for safety.

As you enter the interchange, the right and left sides of the road cross over each other at a stop light. You are, in fact, driving on the left side of the road at this point. From there, left turns become a lot easier, because there's no oncoming traffic in the way. Instead of waiting for a signal, you get a free left turn.

"When we do the cross-over to the left side of the road, that's when the left turns happen, so the left is very easy," says Chlewicki.

That means diverging diamond interchanges can be both more efficient and safer than conventional intersections with left turn lanes. There are now more than 200 of them across the U.S., in more than 30 states. But at first, it wasn't easy to convince other traffic engineers.

"Anything different is a hard sell," Chlewicki said. "Safety was the big question."

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Whitney Shefte for NPR

Making left turns safer and more efficient

The first state to install a diverging diamond interchange was Missouri, way back in 2009.

"Part of the thought was, okay, we put it in there, we see how this works," said Stacy Reese, a district engineer with the Missouri Department of Transportation. "We were willing to take that risk."

The state put the first diverging diamond at a notoriously traffic-clogged intersection in Springfield where it could often take as long as 20 minutes to make a left turn.

When MODOT opened the new intersection, those backups cleared up almost immediately. Reese said. And that wasn't the only benefit: It was also safer than the traditional intersection it replaced.

"We did see the crashes reduced somewhere in that 40 to 50% range, pretty much instantaneously," she said.

Left turns are a big problem everywhere. They have a lot of what traffic engineers call "conflict points," with pedestrians as well as other cars. The diverging diamond design eliminates some of those conflicts, lowering the risk of side impact or T-bone crashes, which tend to be especially deadly.

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Whitney Shefte for NPR

Drivers have strong feelings

Still, some drivers say the unconventional interchange makes them nervous.

"I hate it," said Logan Wilcox, who drives a school bus in a community near the double diamond interchange in northern Virginia.

"I feel like someone that's not familiar with it is gonna be coming through, and they're gonna struggle with it," Wilcox said. "It makes me really concerned that someone's gonna hit me at any point with my bus full of children."

But other drivers at a local gas station like the design.

"It's fantastic. Less aggravation, less accidents. Love it," said Greg Peterson of The Plains, Va. "Best money they ever spent doing that."

"For this type of intersection, it works really well because the traffic flows," agrees Cynthia Dodson of Marshall, Virginia.

A lifelong passion for road design

Reactions like those are gratifying for inventor Gilbert Chlewicki. In a sense, he's been preparing for this career for most of his life.

"I was drawing roads as a little kid," Chlewicki said. "I would draw lanes wide enough from my Hot Wheels, and I would just use a map to kind of guide me on what I wanted to draw."

Chlewicki had the idea for the diverging diamond interchange when he was in graduate school at the University of Maryland. He presented the first major paper on it at a conference in 2003.

There had been some similar road designs before — including, notably, a highway interchange in Versailles, France that Chlewicki visited a few months after his initial insight. But he coined the name diverging diamond, and then pushed to make it a reality across the U.S.

/ Whitney Shefte for NPR

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Whitney Shefte for NPR

The design caught the attention of engineers at the Federal Highway Administration, who threw their support behind it.

Twenty years later, Chlewicki's idea has crossed over into the mainstream. And while two decades may seem like a long time, he doesn't see it that way.

"Honestly, for government and for complex things like interchanges, this went super fast," he said.

Chlewicki is now employed by the Virginia Department of Transportation. He's experimenting with some new "mutations" of the diverging diamond, as he puts it, combining them with roundabouts and other innovative traffic design — still playing with model cars and paper.

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