A new 'diverging diamond' interchange at I-43 and Brown Deer Road will open soon

Southeastern Wisconsin's first diverging diamond interchange will soon open at Interstate 43 and Brown Deer Road as part of the state's reconstruction project to update a significant stretch of the 50-plus-year-old interstate.

The new interchange , including the ramps at Brown Deer Road, are scheduled to open by 10 a.m. on Saturday, Nov. 23, although the pedestrian crossing will remain closed until late 2024 due to continued construction in the area, according to a Nov. 12 update from the Wisconsin Department of Transportation.

The diverging diamond interchange, or DDI, is part of WisDOT's larger I-43 North-South reconstruction project on a 14-mile stretch of I-43 in Milwaukee and Ozaukee counties between Silver Spring Drive by Bayshore Mall and Washington Street (Highway 60) in Grafton.

In total, the project will expand I-43 from four to six lanes, reconstruct five interchanges along the corridor, add a new interchange at Highland Road and replace the Union Pacific railroad bridge over I-43 in Glendale. It will also expand a section of Port Washington Road in Glendale from two to four lanes.

What are diverging diamond interchanges and how do they work? A DDI is a type of highway interchange that WisDOT said makes it easier for drivers to turn left onto the I-43 entrance ramps, according to an information video WisDOT posted on Nov. 9.

In a DDI, traffic on the local road, in this case Brown Deer Road, is briefly shifted to the opposite side of the road at a controlled traffic signal, as shown in a WisDOT diagram.

The crossover allows vehicles turning left onto the I-43 entrance ramps to make smooth, free flowing left turns without crossing in front of oncoming traffic.

Drivers on Brown Deer Road who are turning right onto an I-43 entrance ramp will proceed through the interchange as they normally would, following the flow of traffic.

As vehicles move through the interchange, they approach a second traffic signal at which point they are shifted back to their respective sides of the road.

Previously, the interchange was what's known as a cloverleaf configuration, where a series of loops and ramps guide left-turning movements without any traffic signals or stop signs, as explained on an informational site outlining the differences between cloverleaf and DDIs from Kentucky's Transportation Cabinet.

Why does WisDOT say the diverging diamond design will improve traffic and safety at the Brown Deer Road interchange? While the old cloverleaf interchange may look simpler, the new DDI design will improve traffic flow onto the interstate and increase safety around the area, particularly pedestrian safety, according to WisDOT's informational video.

The interchange has significant current and projected traffic turning left onto I-43, according to the video.

During peak hours, an average of 1,100 vehicles are expected to merge onto I-43 southbound, 800 vehicles onto I-43 northbound and 300 vehicles will continue along Brown Deer Road through the interchange, according to the video.

An informational document from the U.S. Department of Transportation explained that DDIs require fewer lanes to handle the same amount of traffic. This means less right-of-way is needed, which gives drivers fewer conflict points, reduced congestion and fewer impacts when effective.

WisDOT estimates that DDIs reduce the ways vehicles can collide by nearly 50% and eliminate the most severe types of crashes, according to the Nov. 12 update.

Additionally, pedestrians at the Brown Deer Road interchange will be able to use dedicated crossings to more safely cross traffic to a center walk protected by concrete barriers.

Are there other diverging diamond interchanges in Wisconsin? This is Southeastern Wisconsin's first DDI, but the state already has several, according to WisDOT's website :

I-39/90 and Highway 11/Avalon Road interchange south of Janesville

Highway 441 and Oneida Street interchange in Appleton

I-39/90 and Highway 26/Milton Avenue interchange in Janesville

I-39/90 and Highway 81 interchange in Beloit

The DDI was first employed in France in the 1970s, according to Virginia's Department of Transportation , which has also since added a DDI.

The design has gradually been added to interchanges in the U.S. since 2009 when Springfield, Missouri, became the first state to add one at Kansas Expressway and Interstate 44.

Contact Claudia Levens at Follow her on X at .