Coralville's Diverging Diamond Interchange Relieves Congestion

Challenge: The City of Coralville and Iowa DOT partnered to improve the I-80 and 1st Avenue Interchange. The interchange is the gateway to the City of Coralville’s actively developing business and entertainment district, the Iowa River Landing (IRL). The IRL is 180 acres of mixed-use land located in the southeast quadrant of I-80 and 1st Avenue. It is home to the newly constructed University of Iowa’s Hospital and Clinics, a 6,000-seat arena and field house, a hotel conference center, a multi-modal transportation facility, and numerous shops and restaurants with the potential for more development to occur in the next 10-20 years.

The Coralville Diverging Diamond Interchange (DDI) is replacing the existing diamond interchange and is designed to handle the higher projected traffic volumes. One of the biggest safety and congestion concerns is tied to the future number of left turns at the interchange.

Interchange Design

HR Green’s transportation team was hired to design the Coralville Diverging Diamond Interchange and evaluate grade-separated trail alignments to provide safe, direct connectivity for pedestrians through the interchange and across the Interstate to the multi-modal center and the various services and amenities at the IRL. HR Green evaluated 12 different trail alignments and collaborated with emergency services, bicycle coalition, city departments, and the Iowa DOT to determine an alignment that addressed the challenging topography and met the needs of various community interests.

The design of the project includes a unique grade-separated pedestrian trail, underpasses, and a trail bridge over I-80. The project also includes parallel acceleration and deceleration lanes on I-80 and accommodates the potential future widening of I-80. Due to the complex staging of the interchange and the need to maintain at least two lanes of traffic on I-80 during daytime hours, this design includes the construction of two independent bridge structures to replace the existing bridges over I-80 in addition to the pedestrian bridge.

The design also includes an intersection at 11th Street/Iowa River Landing Place, retaining walls, storm sewers, ditching/ culverts, traffic signals, and pedestrian underpasses. Aesthetic design elements are being incorporated to match the City of Coralville’s and Iowa DOT’s I-80 and 1st Avenue master plan.

Traffic Operations

The HR Green’s traffic team provided permanent traffic signal timing plans and temporary traffic signal timing plans for the construction of four traffic signal systems along the 1st Avenue corridor, which includes the interchange with Interstate 80. The team also provided ongoing construction support to maintain traffic operations on both the I-80 and 1st Avenue corridors while the interchange was being reconstructed, with the goal of maintaining the safety and efficiency of traveling motorists.

The interchange is located near the Iowa River Landing district and several University of Iowa facilities, which results in high traffic demands during peak hours in the AM, mid-day, and PM periods. The HR Green traffic engineering team worked closely with the Iowa DOT, City of Coralville, and the prime/sub contractors to monitor and adjust the traffic signal timing parameters to best serve the varying construction phases and traffic control configurations.

To assist with monitoring traffic operations, the HR Green team integrated with the City of Coralville’s Axis Pan-Tilt-Zoom CCTV Camera network and the Iowa DOT’s intelligent work zone CCTV camera coverage. They also helped the City of Coralville update traffic signal timing parameters within the Siemens Tactics Advance Traffic Management System software.

Benefit: By enhancing safety and reducing congestion, the interchange facilitates smoother traffic flow, particularly addressing the future surge in left turns at the junction. Moreover, with improved pedestrian connectivity and thoughtful trail design, the project fosters accessibility to key amenities and businesses in the Iowa River Landing district, promising a more vibrant and accessible community hub for years to come.