Winner of 2016 ACEC Award (State Honor Award & National Recognition Award)
2015 Snapshot:
When rapid growth around Marion began overwhelming the existing I-57 interchange, the conventional solution would have been a larger, more expensive interchange expansion. Instead, TWM determined that an innovative concept, a Diverging Diamond Interchange (DDI), would be an ideal solution.
The first U.S. DDI had only opened a few years earlier in 2009 at the Interstate 44 and State Route 13 interchange in Springfield, Missouri, and very few existed nationwide when TWM began evaluating the concept for Marion. Working with the City of Marion and IDOT District 9, TWM studied traffic operations, future development patterns, and long-term mobility needs along The Hill Avenue corridor. After extensive modeling and evaluation, the team determined that the DDI configuration would improve traffic flow, reduce congestion, enhance interstate access, and support future commercial growth—all at a lower cost than traditional interchange designs.
   
The project ultimately became the first DDI in Illinois to open to traffic, transforming how motorists moved through one of southern Illinois’ fastest-growing retail and recreation corridors. Today, more than 200 DDIs operate across the United States, but Marion’s interchange helped demonstrate the effectiveness of the design long before it became more widely adopted.
To further reduce congestion, TWM redesigned westbound IL Route 13 access with collector-distributor roadways that allowed travelers direct access to The Hill Avenue area without navigating the heavily congested Route 13 interchange.
Beyond traffic operations, the project required extensive multidisciplinary coordination. TWM completed nearly 1.5 miles of survey, traffic modeling for approximately 485 acres of future development, roadway widening studies, bicycle and pedestrian accommodations, three traffic signals, and design of a new four-span bridge over I-57. To keep the accelerated schedule on track, TWM also recommended splitting the work into multiple bid packages, allowing bridge construction and settlement activities to begin earlier while other design components advanced.
What made the project especially unique was not just the engineering solution itself, but the willingness by all stakeholders to champion an unfamiliar concept before it became common practice. Today, DDIs are recognized nationwide for improving traffic efficiency and safety—but in Marion, TWM helped prove the implementation could work long before it became adopted as a standard design.
