Iowa’s Urban Loop (I-35/I-80/IA 141 and Meredith Drive/100th Street Interchanges)
HR Green led a multi-discipline team to improve traffic operations and safety on a critical segment along the combined routes of I-35 and I-80 at IA 141 (locally known as Rider Corner and now known as the Urban Loop) in the Cities of Urbandale, Grimes, and Johnston.
The Urban Loop project begins at the interchange with Douglas Avenue, includes interchange improvements at Meredith Drive, IA 141, NW 100th Street, and ends at the interchange with NW 86th Street. The $87.2M improvement project began with HR Green conducting an Operations Study of the Interstate and local roadway system to identify Purpose and Need and a range of alternatives, feeding into an Interchange Justification Report (IJR) and NEPA Environmental Assessment (EA). HR Green carried the project through Preliminary and Final Design and assisted the Iowa DOT on construction phase services.
History of the Interchange
In the 1970s, the area was rural in nature and northbound traffic from I-35/I-80 could use an exit loop and continue directly north on IA 141 without stopping.
By the 1990s, the area was developed and northbound I-35/I-80 traffic headed to IA 141 navigated a tighter exit loop with traffic signals. Development was also constrained by a lack of good access to the west (red shaded area) that was also restricted by a rail spur line.
As the metro area grew to the north, travel patterns changed to the point where 27% of the northbound Interstate traffic was exiting to the IA 141 corridor, with nearly 90% of that traffic continuing north on IA 141. The existing loop ramp configuration simply could not handle the traffic volumes.
The existing interchange was a partial cloverleaf with an exit loop ramp from northbound I-35/I-80 to IA 141 and a loop entrance ramp from northbound IA 141 to southbound I-35/I-80. The northbound exit loop ramp experienced vehicle queues extending onto mainline I-35/I-80 on a daily basis during the PM peak hour.
The approved concept included removing both loop ramps from the interchange and replacing these access points with a northbound I-35/I-80 to northbound IA 141 flyover ramp as well as a northbound I-35/I-80 exit ramp to Meredith Drive and a southbound I-35/I-80 entrance ramp from Meredith Drive.
The flyover bridge, a continuous welded curved girder design, carries northbound exiting traffic over the rail line, IA 141, I-35/I-80, and other ramps. The horizontal alignment of the bridge is unique in that it includes two reverse curves. The flyover bridge is divided into three units such that each unit has a horizontal curve in only one direction.
The ten piers were located to allow the bridge to span over I-35/I-80 without a pier in the median and to span across IA 141, the railroad, and existing ramps with minimal traffic restrictions during construction. In addition, the piers were located to accommodate the construction of future lanes. The flyover bridge took just under two years to complete and included the development of aesthetic features to complement other bridge projects in the corridor as well as retaining walls, Intelligent Transportation System accommodation, signing, and continuous corridor lighting.
Additional I-35/I-80 access was constructed at the NW 100th Street overpass. A future collector-distributor roadway system will provide additional connectivity throughout the Urban Loop corridor. With the completion of the project, there is a resurgence in economic development as the project brings improved safety and access to the corridor.