At a time when fewer than ten such configurations existed nationwide, TWM performed traffic analysis and determined that a Diverging Diamond Interchange (DDI) was the best solution to rapid development in the City of Marion. Working with the City and IDOT District 9, TWM studied and then reconfigured the existing interchange at I-57 and The Hill Avenue to solve the City’s traffic predicament. The DDI design was the first of its kind to open to traffic in the state and promotes future growth by providing full access to the interstate and improving traffic flow—while costing less than traditional approaches.

TWM’s survey team provided nearly 1.5 miles of topographic survey, covering over 50 acres along roadways and ramps in the project corridor. TWM’s widening study determined which side of the existing 2-lane roadway would be widened and included geometrics layout, typical sections, cross sections, bridge GP&E sheets for both options, and a narrative of the pros and cons for each option. Quantities were also provided to determine the preferred option choice.

Phase I also included traffic generation and modeling of 485 acres, and preparation of seven separate intersection design options and a complete project report. The existing westbound IL Route 13 access to/from I-57 was modified to include collector-distributor roadways to provide access to/from the proposed structure, giving travelers moving north of Marion along I-57 direct access to The Hill Avenue and the adjacent retail and recreation districts without having to travel the congested Route 13/I-57 interchange.

TWM’s structural engineers designed a new bridge over I-57, paralleling the one the firm previously designed in 2008. The new structure is comprised of 4 spans totaling 370’ with a width of 44’. The superstructure consists of 54” PPC I-beams with an 8” thick, cast-in-place, reinforced concrete deck on integral abutments and modified crash wall drilled shaft bent piers.

TWM suggested multiple bid package for the fast-tracked project with the bridge bid out separately, allowing for settlement of the bridge cone. TWM managed the project via an on-call ‘Various’ project with IDOT where 13 separate work orders kept the fast-paced project moving. TWM also managed two DBE sub-consultants.

Scope of Work

  • Design of a bicycle/walking path, widening of The Hill Avenue, and a 4-span bridge over I-57

  • Three traffic signals including two “cross-over” locations


City of Marion, IL / IDOT District 9


Williamson County, IL


  • Experience with traffic control plans for major interchanges
  • Experience implementing ADA-compliant pedestrian facilities