https://www.npr.org/2024/05/28/1243933142/diverging-diamond-interchange-chlewicki Accessibility links * Skip to main content * Keyboard shortcuts for audio player * Open Navigation Menu * NPR logo * * Newsletters * * NPR Shop * Close Navigation Menu * Home * News Expand/collapse submenu for News + National + World + Politics + Business + Health + Science + Climate + Race * Culture Expand/collapse submenu for Culture + Books + Movies + Television + Pop Culture + Food + Art & Design + Performing Arts + Life Kit + Gaming * Music Expand/collapse submenu for Music + Tiny Desk + Hip-Hop 50 + All Songs Considered + Music Features + Live Sessions * Podcasts & Shows Expand/collapse submenu for Podcasts & Shows Daily + [morning-ed] Morning Edition + [we_otheren] Weekend Edition Saturday + [we_otheren] Weekend Edition Sunday + [all-things] All Things Considered + [fresh-air] Fresh Air + [up-first] Up First Featured + [podcast-po] The NPR Politics Podcast + [throughlin] Throughline + [trumps-tri] Trump's Trials + [tile-wild-] Wild Card with Rachel Martin + More Podcasts & Shows * Search * Newsletters * * NPR Shop * NPR Music NPR Music * Tiny Desk * Hip-Hop 50 * All Songs Considered * Music Features * Live Sessions * About NPR * Diversity * Support * Careers * Press * Ethics Road planners embrace an unconventional intersection An unconventional design known as the diverging diamond interchange can be both safer and more efficient than conventional left turn lanes. Road planners across the country are switching sides. National When driving on the wrong side of the road is the right way to speed up traffic May 28, 20245:00 AM ET Joel Rose Joel Rose Loading... 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 inventor of this unconventional interchange. "Like, why are you putting me on the other side of the road?" Traffic engineer Gilbert Chlewicki, the inventor the double diamond interchange, at an intersection in Haymarket, Va. Enlarge this image Traffic engineer Gilbert Chlewicki, the inventor the double diamond interchange, at an intersection in Haymarket, Va. Joel Rose/NPR hide caption toggle caption Joel Rose/NPR Traffic engineer Gilbert Chlewicki, the inventor the double diamond interchange, at an intersection in Haymarket, Va. Joel Rose/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." 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. Enlarge this image 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. Whitney Shefte for NPR hide caption toggle caption Whitney Shefte for NPR 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. 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. Loading... 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. In Haymarket, Va., 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. Enlarge this image In Haymarket, Va., 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. Whitney Shefte for NPR hide caption toggle caption Whitney Shefte for NPR In Haymarket, Va., 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. 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. Loading... 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. This diamond divergent traffic interchange is in Virginia. The first state to install a diverging diamond interchange was Missouri, way back in 2009. Enlarge this image This diamond divergent traffic interchange is in Virginia. The first state to install a diverging diamond interchange was Missouri, way back in 2009. Whitney Shefte for NPR hide caption toggle caption Whitney Shefte for NPR This diamond divergent traffic interchange is in Virginia. The first state to install a diverging diamond interchange was Missouri, way back in 2009. Whitney Shefte for NPR The idea caught the attention of engineers at the Federal Highway Administration, who began pushing to make it a reality. 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. * infrastructure * Traffic patterns * diverging diamond * left * Facebook * Flipboard * Email Read & Listen * Home * News * Culture * Music * Podcasts & Shows Connect * Newsletters * Facebook * Instagram * Press * Public Editor * Corrections * Contact & Help About NPR * Overview * Diversity * NPR Network * Accessibility * Ethics * Finances Get Involved * Support Public Radio * Sponsor NPR * NPR Careers * NPR Shop * NPR Events * NPR Extra * Terms of Use * Privacy * Your Privacy Choices * Text Only * Sponsor Message Become an NPR sponsor