How will the Diverging Diamond Interchange in Sioux Falls look when it’s done? We went to another state to find out.

Published: Mar. 30, 2022 at 3:52 PM CDT
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MOORHEAD, Minn. (Dakota News Now) - A major road construction project is underway in Sioux Falls. The 41st Street and I-29 interchange is being redesigned into what’s called a “Diverging Diamond Interchange,” or DDI, for short.

It’s a relatively new concept, but DDIs are quickly popping up in metro areas throughout the nation.

The Sioux Falls DDI project will take two years to complete, but we wanted to know what it will look like when it’s finished. So, Dakota News Now hit the road to find a similar DDI in the region.

We found one in Fargo-Moorhead, off Interstate 94, where the design has improved traffic flow and motorist safety.

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Diverging Diamond Interchange in Moorhead, MN.
Diverging Diamond Interchange in Moorhead, MN.(Dakota News Now)

“We call these alternative intersection controls,” Moorhead Traffic Engineer Jonathan Atkins said. “Instead of the standard intersection that we’ve had around since the 30s and 40s, we’re trying new methods to move traffic more safely and efficiently.”

The Fargo-Moorhead metro area has a population of about 240,000. Comparatively, Sioux Falls’ DMA census is close to 265,000.

Atkins says the growing communities face similar obstacles.

“Traffic continues to increase and we don’t have the space to continually widen roads to accommodate that traffic, so we have to think outside the box, we have to utilize technology and these new designs in order to manage the traffic flow,” Atkins said.

The DDI works by diverting traffic to the opposite side of the road, giving motorists the ability to turn left onto the interstate without crossing traffic.

“It eliminates that delay at the interchanges, which allow for smoother, better movement,” Atkins said.

Since opening this DDI in 2016, Atkins says traffic volume has increased 40%.

“There are huge efficiencies with this design and it really does flow really well,” Atkins said.

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Marcus Calkins tells Dakota News Now about his experience with the Diverging Diamond...
Marcus Calkins tells Dakota News Now about his experience with the Diverging Diamond Interchange in Moorhead, MN.(Dakota News Now)

Marcus Calkins, a Fargo-Moorehead resident, experiences the benefits of the DDI each day during his commute.

“I think, overall, it helps speed up traffic a little bit. It seems like getting on the interstate and everything, it definitely funnels everyone over quicker,” Calkins said.

While it all could seem confusing at first, Marcus says it really isn’t.

“Once you get used to it it’s a lot better,” Calkins said.

In fact, the city says crashes in this area have decreased by 85%, with no fatalities.

“This biggest increase in safety is that when a crash occurs, it is most likely to be a side-swipe versus a ‘t-bone.’”

However, the safety and efficiency didn’t come without some growing pains.

“When it first opened, the signals on the Diverging Diamond weren’t timed very well, and it took me a while to figure out how to get those properly working with each other and with the adjacent intersections,” Atkins said.

Atkins adds, that’s why it’s all actively monitored.

“It’s a system, all three of the signals, the ones on either side of the DDI, as well as the DDI, have to work together otherwise you don’t move the traffic out,” Atkins said.

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This video shows an example of a drive through a diverging diamond intersection. The video is sped up for brevity.

Having gone through it himself, Atkins has a message for drivers in Sioux Falls.

“Right after it opens, give your city staff a chance to figure it out and manage it,” Atkins said. “Don’t give up on it. It will work, and it will work well, and once you get used to it you’ll wonder what you did before.”

The whole process in Sioux Falls will be done in phases. Phase one is currently underway. The completion of the project is expected by the summer of 2024.

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