Morgan Avenue Diverging Diamond Interchange Over I-57
LOCATION: City of Marion, IL
CLIENT: IDOT District 9 / City of Marion
QUICK FACT: First Diverging Diamond in the state of Illinois
The Illinois Department of Transportation selected TWM to study the existing interchange at I-57 and Morgan Avenue in Marion, Illinois, with a goal of improving traffic circulation at the interstate exit. In the middle of the study, IDOT also asked TWM to evaluate the costs and applicability of using a Diverging Diamond Interchange (DDI) design. A DDI is different from a typical diamond interchange configuration in that the two directions of traffic on the minor intersecting roadway cross to opposite sides at the ramp intersections. Therefore, traffic on the freeway overpass or underpass briefly travels on the opposite side of the road from what would be “normal.” The advantage is the ability to remove left-turn movements from the intersections of the ramps and minor roadway creating a free-flow condition for these movements. It also reduces the required signal phasing to two phases, resulting in a more efficient flow of traffic.
Based upon the conclusions of TWM’s study, IDOT decided to proceed with full design and reconfiguration of the intersection into a Diverging Diamond. Rare in the United States but catching on quickly, TWM completed the first of its kind in the state of Illinois.