Progress on the Interstate 35/80 & U.S. 6 Diverging Diamond Interchange Upgrade

Rising Demands & the Need for Change

The exponential growth of communities, particularly in the western suburbs of Des Moines, has been nothing short of remarkable. The population of Waukee, for example, more than tripled between 2000 and 2020. As these communities flourished, so did the demand for transportation infrastructure, leading to increased traffic congestion and a surge in traffic-related incidents.

In response to these pressing challenges, the Iowa DOT joined forces with Snyder & Associates, a trusted traffic and roadway improvements partner, to evaluate and propose practical solutions. The objective was to enhance the transportation network along the U.S. Highway 6 corridor, which stretches westward from the Interstate 35/80 service interchange.

Interchange Justification Report Leads to Diverging Diamond Interchange Recommendation

The Snyder & Associates team created an Interchange Justification Report (IJR) that was crucial in this process by providing a comprehensive framework for meeting the FHWA’s criteria. Given the challenges presented by the developed land and environmentally sensitive areas surrounding the interchange, multiple alternatives were explored. These alternatives ranged from expanding the existing diamond interchange to converting it into a tight diamond, single-point urban, or a diverging diamond interchange (DDI).

The Diverging Diamond Interchange at Hickman Road and Interstate 35/80 will drastically reduce the traffic conflict points.

After careful analysis, the diverging diamond interchange emerged as the preferred option. Snyder & Associates traffic engineers determined it best aligned with the project’s criteria. The DDI interchange is a type of interchange that can improve safety and traffic flow by eliminating the need for left turns across opposing traffic. In a DDI interchange, traffic briefly crosses over to the opposite side of the road before merging back onto the original side.

“The new DDI interchange will be a major improvement for safety and traffic flow at this busy interchange,” said Snyder & Associates Project Manager Nathan Carhoff. “The DDI design eliminates the need for left turns across opposing traffic, which is a major cause of crashes at interchanges.”

Expanding Trail Accommodations: Enhancing Accessibility

This project area contains one of the busiest segments of Interstate Highway in the State of Iowa. I-35/80 between Douglas and University carries over 125,000 vehicles per day, with projections for traffic to grow to over 185,000 vehicles per day by 2050, creating a dangerous barrier between the communities of Clive and Urbandale. Regionally, Hickman Road also carries more east-west traffic from one side of the interstate to the other than any other roadway in the metro. Currently, Hickman Road carries approximately 40,000 vehicles per day along this corridor, with projections for traffic to grow to over 60,000 vehicles per day by 2050.

Providing a safe barrier between pedestrians and bicyclists is critical for encouraging non-motorized transportation options and allowing for safe recreational opportunities for residents of all abilities. To that end, the construction of several new trail sections is included in the project. The first trail section is a 2,200-foot-long, ten-foot-wide, PCC pedestrian trail connection that will begin on the east side of the Lifetime Fitness entrance off Hickman Road. The trail will turn a loop approximately 500 feet to the east to accommodate elevation change. It will move south via a grade-separated underpass tunnel to safely carry pedestrians and bicyclists under the busy Hickman Road corridor. Turning back to the east and south, the trail will run parallel to the southbound I-35/80 entrance ramp and connect with the Clive Greenbelt Trail near Walnut Creek.

Two pedestrian bridges over the interstate onramps will add a multi-modal component to the new interchange.

The next trail section will begin on the north side of Hickman Road at the grade-separated tunnel entrance. It will continue to the east, crossing the southbound interstate exit ramp via a pedestrian bridge. The trail’s path will pass beneath the interstate using a grade-separated underpass tunnel and continue over the northbound entrance ramp via another pedestrian bridge. Ultimately, the trail will extend east, connecting with a local trail and sidewalk network near NW 111th Street.

When complete, this new grade-separated connection will bridge a much-needed gap in the area trail network, providing users with undisturbed access to the Raccoon River Valley Trail and the Clive Greenbelt Trail that collectively tie thousands of residential homes and hundreds of area businesses together, along with dozens of parks and recreational areas — all with minimal to no interaction between trail users and the heavy Hickman Road and Interstate 35/80 traffic.

These trail accommodations align with the project’s broader goals of enhancing accessibility and promoting alternative modes of transportation within the community. They are planned for construction concurrently with the interchange upgrades, demonstrating a holistic approach to transportation infrastructure development.

Enhanced Stormwater Management Planning

The quality of life for users and aquatic life will also be improved by incorporating numerous stormwater management elements to help control and clean up the added runoff associated with the additional pavement areas. Most improvements drain to Walnut Creek located just south of Hickman Road. This urban stream takes on stormwater runoff from thousands of developed lots within the western suburbs of Des Moines, so any additional pavements contribute to higher runoff volumes and quicker discharges to the stream. With faster overland flow, many sediments and water contaminants are also carried within those waters, further damaging the streams and affecting the natural habitat for residents, fish, and plant life.

To address these additional runoff concerns, stormwater within the project area will be treated via grass swales with a rock infiltration sub-base installed in the drainageways. These improvements will promote stormwater infiltration into the ground, and gabions installed at strategic locations will slow the water further, encouraging groundwater infiltration and reducing erosion. Other areas of the project will be treated using bioretention cells. These cells will be located within the median islands of the interchange.

All stormwater management features are designed based on city requirements for recharge and water quality volume. This infrastructure will eliminate damage to property and the environment by reducing, slowing, and controlling the flow of stormwater runoff. Properly managed streams and natural waterways enhance the community’s natural beauty, increasing community pride, and helping make it a place where people want to live, work, and recreate.

Moving Traffic Safety Forward: A Safer and More Efficient Corridor

Rendering of the new interchange looking to the northwest.

As the project to revamp the I-35/80 & U.S. 6 interchange progresses into the construction phase, it promises safer and more efficient transportation for the communities of the Des Moines metro area. With careful planning, adherence to FHWA standards, and a commitment to addressing the region’s evolving needs, this transformative initiative is well on its way to becoming a reality. Stay tuned for further updates as we witness the continued evolution of this vital transportation corridor.