IDOT Selects TWM to Study and Design Innovative DDI Configuration on I-57 at Marion


January 2011 – Eight years ago at a transportation symposium in California, a civil engineer from Baltimore introduced the United States to the concept of a Diverging Diamond Interchange (DDI) – a unique approach to maximizing safe, efficient traffic flow at the intersections of major roadways. At that time, DDIs existed only at a few locations in France. But the Missouri Department of Transportation was intrigued, and in 2009 constructed the first DDI in the U.S. at Springfield, MO. That same year Popular Science magazine dubbed the concept one of the best innovations of 2009.

Missouri has since constructed two more DDIs with more in the planning stage. Utah and Tennessee have each constructed one and a few other states are considering the concept. Now the State of Illinois plans to join this elite group. In January, IDOT asked TWM to proceed with the study and design to reconfigure the existing interchange at I-57 and Morgan Avenue at Marion to a DDI.

A Diverging Diamond Interchange is different from other Diamond Interchange configurations in that the two directions of traffic on the minor intersecting roadway cross to opposite sides at the ramp intersections. That means traffic on the freeway overpass or underpass briefly travels on the opposite side of the road from what would be “normal”. The advantage of the crossover configuration 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. 

At IDOT’s request, TWM presented data for both a DDI and a Conventional Diamond configuration, in which on and off ramps from a bridge form a diamond shape to provide access to or from a crossroad. Both were considered for their minimal impact to the existing interchange. However, the DDI was shown to provide greater capacity than the Conventional Diamond, and by IDOT’s calculation for this particular location, the cost of the DDI is expected to be roughly 28% less than that of the Conventional Diamond.  The DDI is particularly suited to interchanges where traffic to and from the major crossroad is greater than through traffic on the minor crossroad, as is anticipated for the Morgan Avenue project.