As the only interstate link between Milwaukee and its North Shore suburbs and a vital connector to Green Bay, Wisconsin, the Interstate 43 North-South Freeway carries upwards of 85,000 vehicles daily. Traversing two counties and 10 municipalities, both urban and rural, the I-43 corridor runs adjacent to residences, businesses, schools, a Union Pacific railroad bridge and environmentally sensitive areas.

One of Wisconsin Department of Transportation’s (WisDOT) larger projects, the expansion of I-43 from four to six lanes was part of the agency’s Major Highway Development Program, which addresses the highest priority projects on heavily traveled highway segments. Projects within this program are economically significant endeavors that encourage development and improve communities’ quality of life.

“There has long been an appetite for this expansion from members of the public who travel that corridor daily, as well as from businesses and residents along that 14-mile stretch,” said David Nguyen, WisDOT design and construction chief.

HNTB partnered with WisDOT throughout the project’s life cycle, delivering comprehensive services from early environmental documentation and preliminary engineering to final design, construction engineering and inspection, traffic management and public involvement.

Meeting safety goals

Safety, a critical WisDOT project priority, was infused into every aspect of the project. A well-thought-out traffic management plan, which included close coordination with local and freeway law enforcement, ensured that travel flowed smoothly and safely.

Technology played a key role in keeping drivers informed and safe. Dynamic message signs, portable changeable message boards and WisDOT’s traffic management system fed real-time data into navigation apps, alerting drivers to work zones and slowdowns. Combined with a strategy to keep two, 12-foot lanes with 2-foot shoulders open throughout construction, these measures helped reduce crash rates during construction compared to pre-project levels.

At the heavily traveled Brown Deer interchange, where traffic previously had to traverse short weaves through an outdated tight cloverleaf interchange, HNTB designed a diverging diamond interchange with safer, free-flowing left-hand turns. Besides creating safety benefits, this newly designed interchange had a smaller footprint allowing for additional space for stormwater detention and the development of prairie areas for threatened and endangered species.

Maintaining a rail route

The project team worked with Union Pacific to replace a 100-year-old rail bridge that crosses above I-43 and two frontage roads.

“On that key freight route, we detailed a design solution and synchronized staging that allowed the new bridge to be built while the old bridge remained in service and freeway traffic continued to flow underneath,” said HNTB deputy project manager Ryan Janke.

There was close coordination with nearby residents to minimize impacts to their properties during and after the new bridge’s construction. The team communicated about the work that would take place essentially in residents’ backyards and built additional retaining walls along those backyards to keep the slopes of the new railroad offset alignment out of the private properties.

Scaling Up on I-43 North-South Freeway

Mitigating environmental impacts

WisDOT’s commitment to minimizing the projects’ environmental impacts informed the construction plan.

In wetland areas along the freeway, a newly erected barrier on the outside shoulder keeps drivers safe while allowing a steeper slope on the opposite side that reduces wetlands impacts. Flowering native seeds purposely chosen and planted along the corridor enhance the habitat for the area’s endangered rusty patched bumblebees.

Where streams that are home to threatened and endangered species run below the freeway, the design minimizes the crossings’ footprints. A staging plan allowed those crossings to be built in seasons that would least impact flora and wildlife. In one case, the project team built a new clear-span bridge rather than rebuilding an existing box culvert because the bridge construction methods were more environmentally sensitive.

An extensive drainage analysis helped to address low points on the freeway that historically had experienced flooding. This resulted in miles of the highway’s low-lying areas being raised and detention ponds being added to help control runoff. These changes improved the North-South Freeway over its previous state from both environmental and water quality standpoints.

The redesigned I-43 North-South Freeway opened in July 2025 — on time and on budget. Initial safety data shows that crashes on the new roadway have declined by 50%.

“Major projects like this one demonstrate the success that is possible through vision, collaboration and communication,” Kowske said. “The rebuilt corridor has improved safety and mobility, strengthened WisDOT’s relationship with the surrounding communities and prepared the freeway for a long-term future serving a growing region.”