The long awaited “diverging diamond” interchange at SC Highway 290 and Interstate 85 finally opened on Monday.
I had to go check it out for myself. And let me tell you: It’s pretty cool.
It felt a little radical driving on what seemed like the wrong side of the highway on the bridge over I-85.
I got off I-85 southbound onto the ramp leading up to 290 in Duncan. At the top of the ramp was a left turn light on red. But it turned green quickly, and I made a left turn and onto the left side of the road.
That’s the unique thing about the diverging diamond design: It puts drivers briefly on the left side of the highway.
If you’re driving all the way through the interchange – say, from Duncan over to Reidville Road – you would actually cross over to the left side of the road and then cross back to the right side.
It seems weird, but experts say it helps to keep vehicles moving by limiting the volume of traffic at intersections. An SCDOT video shows how it all comes together.
On my ride-through on Monday, traffic seemed to be flowing efficiently. The left turn light turned green quickly, and there was no major backup – this was at close to 5 p.m. I was able to turn left to get back on I-85 with no wait.
The diverging diamond concept is the brainchild of a traffic engineer named Gilbert Chlewicki.
National Public Radio reported that he presented the idea at a conference in 2003 while a graduate student at the University of Maryland.
He told NPR, “(A) lot of people looked at me like I was a little nuts. Like, why are you putting me on the other side of the road?”
Eventually, the idea caught on.
The first was developed in Springfield, Missouri in 2009, according to the Federal Highway Administration. There are now about 150 of the interchanges.
This diamond interchange is the third in South Carolina, according to the SCDOT. The state’s other two are in Fort Mill and the Chapin area.
There’s also one just south of Asheville, off Interstate 26.
Duncan Town Administrator Cameron Fant said navigating the Asheville diverging diamond helped him prepare for the new section of 290.
“The first time I went through, it kind of freaked me out,” he said. “It’s an adjustment going over to the left side of the road. But once you go through it a time or two, you’re a fan.”
On day two of the Duncan diverging diamond, he said things were “going amazingly smooth.”
The project took longer than the SCDOT had planned, and that was frustrating for Fant and for motorists depending on the 290 interchange to get from place to place.
But the result is worth it, Fant said. “Overall, I would give the SCDOT a pat on the back.”
So, when it's your first time approaching a diverging diamond, pay attention, but don’t get too stressed out. As some people might tell you: Trust the process.
Reporter Baker Maultsby covers transportation issues. He has traveled to, from, and around the Upstate for years. Send him transportation-related tips at [email protected]