NO. 5: The Urban Loop - I-80/I-35 & IA 141 and Adjacent Interchanges

The Highway 141 flyover bridge was the cornerstone to resolving congestion.

Located at the convergence of I-80 and I-35 in the northwestern quadrant of Des Moines, Iowa, not only was the flyover a critical element of the overall interchange design, but also for the planning and design of adjacent interchanges.

As the combined routes of I-80 and I-35 curve around the outskirt of Des Moines, the existing interchange with IA 141 featured a folded-diamond interchange, with an exit loop ramp where 90% of the exiting traffic continues north on IA 141, going through several traffic signals before leaving the interchange area.

Traffic often backed up onto the interstate in the northbound direction from the exit loop ramp, resulting in delays, congestion, and crashes. Urbandale officials and leaders from other adjacent communities sought improved access to support and promote economic development.

The design process started with the flyover concept to solve the most critical issue in the corridor, which was the exit loop ramp. “Right sizing” the balance of the solution through context-sensitive design led to the flyover concept, along with adding a partial interchange to the south at Meredith Drive and a full interchange to the east at 100th Street.

A final, or ultimate build, scenario was developed and approved by FHWA to add collector-distributor roads adjacent to I-80/I-35 mainline, connecting the added adjacent interchanges to enhance system operations and performance as traffic patterns evolve and volumes build.

The Urbandale Economic Development organization has now coined the corridor the “Urban Loop – Turning Access into Opportunity,” and with the opening of the new transportation infrastructure, economic growth is accelerating in the area.

David Dougherty, transportation president at HR Green and manager of this project, credited the Federal Highway Administration, the Transportation Research Board, and local DOT engineers with giving his design team good resources to adequately assess design standards and geometric layouts.

“During the concept development phase and into preliminary engineering and final design, there was a constant assessment of both the cost of the infrastructure for the design itself and the cost of impacts to right-of-way and adjacent infrastructure such as adjacent interchanges, city streets, parking lots, city utilities, drainage, etc. to keep the scope of the project in check,” he said.

“In the early stages of project development, we sought to develop a single build alternative that provided elements of the ultimate, parallel collector-distributor roadway system. However, anything short of the simple, straightforward collector-distributor concept resulted in complex, confusing geometry or a scale of project beyond what was feasible at this time.”

People often say the simplest solution is usually the best for a reason, and that’s why the “right sizing” philosophy worked so well for this project, according to Dougherty.

“The team worked to the initial build and ultimate build strategy to meet current needs while providing expandability and adaptability to future traffic and travel pattern conditions,” he added.

The right sizing effort applied to the entire project resulted in a future configuration featuring collector-distributor roadways parallel to I-80/I-35 connecting adjacent interchanges and allowing weaving of traffic to occur off mainline. 3-D modeling was utilized to examine the future geometry and adapt the current project to the future conditions. Another future element is at the 37th arterial street intersection, north of the flyover on IA 141.

Recognizing the heavy traffic volumes through this intersection and the predominant northbound traffic pattern, a grade separation over 37th Street concept was developed and approved by FHWA to keep flyover traffic destined to locations north of 37th Street separate from local traffic at the intersection and further alleviate queuing risk to the free-flow nature of the flyover.

The City of Grimes also independently studied a future interchange at 37th Street that could be accommodated within the IA 141 interchange concept. This approach kept the project budget within programming limits to achieve critical goals now, while providing for future expandability.