Dimensions - Summer 2009 - 4

The Diverging Diamond Interchange At the intersection of Interstate 44 and Route 13 in Springfield, Missouri, travelers are now driving the Diverging Diamond Interchange (DDI), reportedly the first in the United States. The Missouri Department of Transportation (MoDOT) developed the design documents and supervised construction of the interchange based on a DDI concept first introduced by Gilbert Chlewicki, PE. Dimensions talks with Chlewicki, a transportation engineer and planner in Dewberry’s Baltimore office, about the interchange and its unique configuration. In the simplest of terms, what is a DDI? A DDI looks very similar to a standard diamond interchange except the cross street with the signals have crossover movements at the ramp terminals. This means that when driving between the ramps, a driver will be driving on the left side of the road. The geometry allows drivers to make a direct left without crossing opposing traffic, which improves the interchange’s efficiency. A Q&A on the Interchange’s Dramatic Efficiencies What was your inspiration for the unique geometry? In 2001, I was investigating interchange concepts that were used for intersections, like the jughandle. I started sketching some other possibilities, when I developed an idea called the Synchronized SplitPhasing Intersection, also known as a double crossover intersection. (This is explained in my paper “New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange.”) I converted concepts of this unique design into an interchange, creating the DDI. About eight months later while on an excursion from Paris to Versailles, I was shocked to discover our bus going though an existing DDI. I was happy to see the concept in practice but disappointed to see that my design was not completely original after all. Have you made modifications to your original design? Yes, some deal with specific design elements to make it even more efficient while others address adding elements that can make a DDI work for multiple scenarios. One of these elements is an inexpensive way of adding a through movement for the ramps. I have also begun investigating the conversion of nondiamond interchanges into DDIs. What are the DDI’s cost and safety benefits? The beauty of this design is its low-cost efficiencies. Because left turns from the arterial do not need an exclusive lane, fewer lanes are needed than other interchange forms. Fewer lanes means less construction and little to no bridge work. The cost savings can be as much as 70 percent compared with other interchange options. The DDI could save states millions of dollars, while significantly improving traffic flow. With the number of conflict points in a DDI cut almost in half as compared to a standard diamond, the interchange presents fewer opportunities for crashes. Simulation tests and observations from the Springfield DDI show that drivers experience little to no confusion driving through the interchange. MoDOT has published Springfield DDI expectations for safety, efficiency, and economy. • Interchange Reconfiguration Construction Time: 6 months, not 1.5 years to 2 years • Construction Cost Estimate: $2.9 million, compared to a standard interchange cost of about $8.5 million • Safety Projections: Reduced rear-end crashes on the bridge and right-angle crashes turning onto Interstate 44 View videos of the interchange and read Chlewicki’s paper, “New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the DDI.” Image courtesy Cathy Morrison and Missouri Department of Transportation 4

http://www.urbanstreet.info/2nd_sym_proceedings/Volume%202/Chlewicki.pdf http://www.modot.mo.gov/springfield/major_projects/Greene/documents/I-44-Route13DDI.pdf http://www.dewberry.com/news.asp?id=128

Dimensions - Summer 2009

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