Taking a look at the safety benefits of diverging diamond interchanges


SIOUX FALLS, S.D. (KELO) — Something new is coming to South Dakota’s two largest cities.

The Department of Transportation is installing a different kind of intersection, called a diverging diamond, in both Rapid City and Sioux Falls.

The first diverging diamond is already under construction in Rapid City on Lacrosse Street over I-90
It should be finished sometime in about two years.

The second diverging diamond will be constructed in Sioux Falls on 41st street over I-29.

Here’s how it works: while east bound traffic is stopped at a red light, vehicles heading west on 41st Street, will diverge from the right side of the road to the left side as they go over the interstate, then back to the right side of the road.

Not only will a diverging diamond handle a lot more traffic it will make making that left hand turn onto the interstate a whole lot safer.

“Here at 41st Street I-29 we have a ton of left turn movements to get onto the interstate system, with the left turns there are a lot of crashes, a lot of rear end crashes, a lot of left hand crashes, and diverging diamonds have been statistically determined to reduce crashes by over 50 percent,” Sioux Falls traffic engineer Shannon Ausen said.

The diverging diamond may look intimidating, after all its sends traffic into what appears to be the oncoming lane.

But Ausen says they plan to make it easy for drivers.

“I’ve driven through a couple of them now, there are so many concrete barriers that are channelizing the drivers on where they’re supposed to go, we rely on those concrete barriers to make it an easy way for drivers to figure out where they’re going, vs just putting pavements markers down on the concrete,” Ausen said.

The 41st Street project is a joint effort between the city and the state. Travis Dressen, a DOT engineer, says when they first started to look at diverging diamonds there were about 60 across the country.

“That number has grown now to over a 100, and we are seeing that these things really handle high traffic volumes, we know 41st Street is that, especially in the morning we have a lot of that traffic trying to make a left hand turn to get on the the interstate, so it really handles those left hand movements very well,” Dressen said.

Dressen says by 2025 the Sioux Falls area will have 4 diverging diamonds. The others will be located at Benson Road and I-29. The Brandon exit on I-90. And at 85th and I-29, when approved.

Dressen says they are always looking at ways to improve traffic efficiency and safety and these projects in Rapid City and Sioux Falls check both boxes.

Both the Rapid City and Sioux Falls projects are sizable and complicated, so they are expected to take about two years to complete.

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