Road Planners Embrace the Diverging Diamond Interchange To Speed Up Traffic (npr.org) 200
schwit1 shares a report from NPR: 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?" 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." In 2009, Missouri became the first state to install a diverging diamond interchange (DDI) at a congested intersection in Springfield. This new design quickly reduced traffic congestion and significantly improved safety, with crashes decreasing by 40-50%.
However, drivers have mixed feelings about the design. Some, like school bus driver Logan Wilcox, feel it can be confusing and potentially dangerous for unfamiliar drivers. Others, like local driver Greg Peterson, praise it for improving traffic flow and reducing accidents.
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." In 2009, Missouri became the first state to install a diverging diamond interchange (DDI) at a congested intersection in Springfield. This new design quickly reduced traffic congestion and significantly improved safety, with crashes decreasing by 40-50%.
However, drivers have mixed feelings about the design. Some, like school bus driver Logan Wilcox, feel it can be confusing and potentially dangerous for unfamiliar drivers. Others, like local driver Greg Peterson, praise it for improving traffic flow and reducing accidents.
