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Client Name
City of Ankeny & the Iowa DOT
Client Type
Services Provided
  • Bridge Concept & Feasibility Studies
  • Bridge Design
Project Manager
Contact Steven Kunz, PE
Structural Engineer

July 2020 Progress Update

Benefiting from favorable construction weather, the Snyder & Associates’ designed Diverging Diamond Interchange at Interstate 35 and East 1st Street in Ankeny is experiencing rapid progress of late. The new southbound I-35 bridge is almost complete and concrete is being poured along the redesigned East 1st Street corridor on an almost daily basis. To the east of the interchange, the new bridge over Fourmile Creek has also been completed and preparations are being made to pour concrete for the roadway approach. If this good weather continues, the Iowa DOT is anticipating the new interchange will be open to traffic in November of 2020.

Check out our latest progress video that provides a bird’s eye view of this expansive project site.

Explosive Growth Creates Opportunities for Improving Interchange

Extending from the northern to the southern border, Interstate 35 bisects the State of Iowa and serves as a primary transportation corridor connecting the state with other locations throughout the Midwest. When I-35 was constructed in 1965, the City of Ankeny was barely a blip on the map with just over 1,000 people. Straddling the interstate on the north side of Des Moines, Ankeny has since ballooned into an almost 70,000-strong thriving community — and continues to grow to this day.

The residential and commercial development that’s accompanied the area’s growing population has also led to an increase in traffic. With a limited number of interchanges moving traffic into and out of the community, intensive traffic congestion problems began to mount, particularly at the East 1st Street interchange, where severe crashes were becoming all too commonplace.

A study completed by the Iowa DOT, in conjunction with City of Ankeny personnel, proposed the following improvements:

  • Construction of a Diverging Diamond Interchange (DDI) at the East 1st Street location
  • Widening I-35 from six to eight lanes, extending from Oralabor Road to East 1st Street
  • Widening I-35 from four to six lanes from East 1st Street to NE 36th Street
  • Widening East 1st Street to five lanes, extending from Delaware Avenue to Frisk Drive to accommodate the DDI configuration and provide additional road capacity for continued growth
  • Reconstruction of two I-35 bridges over Fourmile Creek, two I-35 bridges over East 1st Street, and the widening and overlay of the East 1st Street bridge over Fourmile Creek, just east of the proposed DDI

The Snyder & Associates team served as the primary design consultant for roadway and interchange ramp improvements, while our subsidiary Shuck-Britson, Inc., handled the lead structural design for all bridges and box culverts associated with the project. Snyder & Associates also facilitated coordination with utility owners and developed the required platting and necessary permits. Our team contracted with Terracon, Inc. to complete all geotechnical engineering. During construction, a detailed staging plan was implemented to reduce traffic disruption and complex detours, while also maintaining the safety of the public and contractors.

Reduced Environmental Impact & Increased Efficiencies Weigh on DDI Decision

An environmental assessment completed in 2015 concluded this project wouldn’t result in significant social, economic, or environmental impacts that would further warrant an environmental impact statement. However, because Fourmile Creek is a sensitive urban stream, culverts and bridges were designed in accordance with DNR and DOT requirements to reduce upstream flooding issues.

A Diverging Diamond Interchange was chosen over other alternatives for its ability to safely and efficiently move large volumes of traffic while minimizing its footprint on the surrounding area. Fewer conflict points also result in fewer opportunities for crashes. Construction on this expansive project began in 2017 and is expected to conclude in 2020. When completed, the East 1st Street DDI will be the second of its kind in the State of Iowa.