I-90 Exit 406 at Brandon interchange nears completion

For as much road construction Brandon commuters have had to deal with, they’re about to get some relief.

According to Harry Johnston, an engineer with the South Dakota Department of Transportation, Exit 406 interchange on Interstate 90 (Brandon exit) is on schedule to be open and functional by the end of the year.

Although the entire two-year project won’t be 100 percent completed until next spring when crews will return for sporadic clean-up and touch-up grass reseeding, Johnston said the diverging diamond configuration could likely be done by the end of October.

Michael Noonan, the project manager on the diverging diamond, said two remaining light signals need to be constructed on top of the bridge.

Other large projects still remaining include the completion of the eastbound railroad bridge, just west of the interchange.

“That’s probably 50 percent complete,” Noonan said. “At the actual interchange, we just finished the bridge deck, so all of the tie-in pavement needs to be completed. But I don’t see an issue with all of the islands being completed this year so we’ll be into full traffic.”

Exit 406 will be South Dakota’s fourth diverging diamond interchange. There is one in Rapid City, and two in Sioux Falls at 41st Street and Interstate 29 near the Empire Mall. The other is at Benson Road and Interstate 229 on the city’s north side. 

A third one is scheduled for Sioux Falls at a new interchange at 85th Street and Interstate 29 on the south side. That interchange hasn’t been built yet, but Johnston said they are beginning the bid process on that project later this fall.

A diverging diamond may take commuters some time to get comfortable with. While crossing the bridge, cars travel on the left side of the road, a strange feeling as drivers are always on the right.

Despite the awkward approach at first, Johnston said multiple studies have been shown a diverging diamond interchange is the most efficient and safest. 

“The biggest thing is it reduces the conflicting traffic movements,” Johnston said. “It’s going to minimize those left turns where you need to cross lanes of traffic so it improves the safety overall and the traffic flow through the intersection.”

Johnston said traffic accidents have dropped substantially at the diverging diamond in Rapid City, as well as the two currently in Sioux Falls.

“It should help with safety. We collect and watch those numbers closely and we’ve seen some great numbers,” Johnston said. “We haven’t seen the accidents, so we’re hoping that all of this is going to be great. There’s over 100 of these nationwide, and every state has that same reduction in crashes.”

As confusing as it may sound to drive on the ‘wrong’ side of the road, experiencing it is a very easy exercise. Johnston said with light signals, directional lanes and retaining walls, it makes it nearly impossible to make a mistake.

“You’d have to physically try to drive it backwards,” he said.

The project is funded by the state, but the city of Brandon is on the hook for a portion of the project, such as moving utilities and local traffic signals. City engineer Tami Jansma said there were no surprises during construction and the city will be able to complete their portion of the project for what was budgeted.

Jansma, too, said the diverging diamond is the correct type of intersection for Exit 406, especially with the future growth projected for the area between Sioux Falls and Brandon.

“There’s been a lot of studies, and it’s the best way,” she said. “That left hand turn onto the interstate in the morning, that’s the part that’s really needed. With a diverging diamond, it gives that left turn more of a priority. It should help traffic that way so it doesn’t back up so much.”