SIOUX FALLS, S.D. (KELO) — In less than a week construction will start on a new way to enter and exit Interstate 29 from 41st Street in Sioux Falls.

State and city officials have said for several years that the diverging diamond interchange at 41st Street and I-29 will reduce crashes and create a safer way to exit and enter the interstate. The diamond eliminates left-hand turns across lanes of incoming traffic. The entire project will also affect traffic flow on nearby streets and intersections.

The city’s website said that this year, crews will focus on the bridge structure and updates to 41st Street from Carolyn Avenue to Shirley Avenue. Work will start first on the bridge. Traffic will be reduced to one open lane in each direction with a center turn lane across the bridge.

More than 25,000 vehicles a day use the area of the I-29 east and west ramps at 41st, according to traffic data from the city of Sioux Falls. A similar amount uses the I-29 west ramp area at Terry Avenue.

A 2017 analysis by the South Dakota Department of Transportation said that 78% of the crashes at the intersection of 41st Street and I-29 southbound were rear-end incidents and 17% were angle incidents.

At the I-29 northbound intersection, 53% of the crashes were rear-end incidents and 40% were angle incidents.

A study completed for the Missouri Department of Transportation in 2011 noted that crash rates declined by 28% during the first year after a diamond diverging interchange construction when compared to the average across a five-year baseline. The study analyzed first-level crashes. “Total crashes were down when comparing the average across the five-year baseline to the one-year post-construction period by 46%,” the study said.

A diverging diamond interchange study on an interstate in Kentucky concluded, “Examining crash types, it is clear that DDI installation should mean a substantial reduction of angle and
turning crashes and a reduction in rear-end crashes.” The study was released by the Federal Highway Administration and North Carolina State University. The study also concluded that sideswipe crashes may remain steady or possibly increase with a diverging diamond.

Diverging diamond interchanges address the interchange at the interstate with the street but also the impact on arterial feeder streets such as Marion Road in Sioux Falls.

The project is referred to as the diverging diamond project but it involves more than the 41st Street exits and entrances on I-29. Traffic on arterial roads in the area can back up as vehicles are waiting to enter the interstate, or while they cross the interstate.

Work is scheduled on the Shirley Avenue and 41st area this summer. Shirley Avenue from the Mall Drive had an average daily traffic count of 24,600 in 2022, according to the city data.

The 2017 analysis included a study of crashes in the Shirley Avenue area.

The area including the intersection of Marion Road and 41st Street was reconfigured. 41st Street between Valley View Road and Marion Road gets roughly 21,000 vehicles on average per day, based on 2018-2021 data. The average traffic flow from Terry Avenue on 41st Street to I-29 west ramps was about 28,000 vehicles per day, according to city data.

According to the 2017 DOT study, 53% of the crashes at 41st Street and Marion Road were rear-end crashes while 37% were angle crashes.

Those are the type of crashes studies and officials said can be reduced with a diverging diamond interchange at I-29 and 41st Street. Congestion can contribute to rear-end or angle crashes.

The SD DOT said diverging diamonds will reduce congestion. The North Carolina Department of Transportation has also cited a reduction in congestion with diverging diamonds.

Construction started at I-29 and 41st Street in 2022. It’s expected to end in 2024.