Earlier this year, the Commonwealth of Virginia opened its fifth diverging diamond interchange. Set over Interstate 95 in grow- ing Stafford County, the new interchange addresses congestion in the county’s busy Courthouse Road area and builds upon the Vir- ginia Department of Transportation’s (VDOT) successful track record with this innovative interchange configuration. The Benefits of the Diverging Diamond Interchange Diverging Diamond Interchanges: A Coast-to-Coast Trend By Steve Kuntz, PE, DBIA, and Howard Zabell, PE
The new DDI over Interstate 95 in Stafford County, Virginia, is the fifth that VDOT has opened in the state. Photo: Dewberry
other configurations that were considered. VDOT recognized that the DDI configuration would handle traffic projections and reduce the footprint of the project. VDOT selected Dewberry as part of its design-build team to design the replacement of the existing diamond interchange that ran under I-95 by realigning Route 630 approximately 800 feet to the south and configuring the new interchange with an overpass of I-95. The project encompassed two and a half miles of roadway widening and recon- struction on Route 630 and included a park and ride lot with more than 1,000 spaces, along with bus shelters, motorcycle spaces, slug line accommodations, bus drop-offs, and pick-up accommodations. Educating the Public: A Vital Step in the Process Although Dewberry had developed conceptual plans for several di- verging diamond interchanges, the I-95/Route 630 DDI represented the first interchange that proceeded through full construction plans. Our team recognized that an extensive public outreach campaign would be essential to inform the public about how the interchange works and al- lay concerns. Some residents and commuters expressed fear of driving down the “wrong” side of the road, turning incorrectly, or accidentally not paying attention to the signage. Together with VDOT, we created a variety of educational media and exhibits that included fly-throughs, renderings, and animations. VDOT also incorporated training for the interchange into the driver’s education program at a nearby high school. In a unique outreach effort, our team created two movie trailers that were showcased at local theaters in Fredericksburg prior to the interchange opening so that people could familiarize themselves with the interchange operations. We conducted several public information meetings for the communi- ties along the corridor to share the impacts associated with the high- way widening and the functions of the new interchange. This effort included a large 50-foot directional mat on the floor of a local high
More than 100 of these “double crossover” interchanges have been built in the United States over the past dozen years, clearly demonstrating the utility and value of this solution. A diverging diamond interchange, also known as a DDI, temporarily crosses traffic to the opposite side of the road, enabling vehicles to travel directly onto interchange ramps and eliminating conflict points associated with left turn movements at conventional diamond interchanges. There are a number of benefits to this newer configuration as opposed to conventional solutions, including cloverleaf interchanges, direc- tional or semi-directional flyovers, partial interchanges, single point urban interchanges, and diamonds. Diverging diamond interchanges can be effective by: • Reducing the number of conflict points over a conventional dia- mond interchange • Addressing intersections where through volumes remain low but turning volumes are high, and where travelers who need to make a left turn have difficulty finding gaps in traffic flow • Cost-effectively modifying existing interchanges by constructing cross- overs, shifting traffic to the opposite side, and then reusing the bridges and interchange ramps. This extends the operational life of an inter- change and can result in significant savings—a few million dollars for the construction of interchange modifications versus tens of millions for a completely new interchange. Making the Decision to Go with the Diverging Diamond Approach For the latest diverging diamond interchange in Virginia, known as the I-95/Route 630 Reconstruction and Widening project, VDOT made the decision to use this configuration early on. Here, the determination was not based on extending the service life of the interchange but because it required less right-of-way and would create less of an impact than
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csengineermag.com
october 2020
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