Diverging Diamond Interchanges: A Coast-to-Coast Trend

By Steve Kuntz, PE, DBIA, and Howard Zabell, PE

Earlier this year, the Commonwealth of Virginia opened its fifth diverging diamond interchange. Set over Interstate 95 in growing Stafford County, the new interchange addresses congestion in the county’s busy Courthouse Road area and builds upon the Virginia Department of Transportation’s (VDOT) successful track record with this innovative interchange configuration.

The Benefits of the Diverging Diamond Interchange

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, directional 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 diamond 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 crossovers, shifting traffic to the opposite side, and then reusing the bridges and interchange ramps. This extends the operational life of an interchange 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 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 reconstruction 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 diverging 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 allay 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 communities along the corridor to share the impacts associated with the highway widening and the functions of the new interchange. This effort included a large 50-foot directional mat on the floor of a local high school that visitors could walk along, pretending they were in a car and mimicking the motions of the diverging diamond interchange. The exhibit included graphics of all of the highway signs so that visitors could see what the updated signage would look like, where the traffic signals would be, and how it would operate.

California’s First Diverging Diamond Interchange

On the West Coast, construction is underway on California’s first diverging diamond interchange for the City of Manteca’s Union/SR120 Interchange Project in Manteca. Here, city administrators determined that the configuration would be the optimal solution to improve its State Road 120/Union Road interchange. Designed by the consulting firm of Mark Thomas and Company and built by local contractor Teichert Construction, the interchange will significantly reduce potential accident locations and improve traffic flow. The existing interchange has 26 potential conflict points while the DDI design reduces this to 14. Dewberry has provided construction management services for the project.

Staging is critical to the success of diverging diamond interchange projects, and this has proven to be the case with the construction in Manteca. The need to relocate overhead utilities delayed the project by approximately six months. Staging can be complex where alignments overlap, requiring a significant amount of field and public coordination, which was in this case complicated by the utility delay. Additional staging challenges in the electrical work, drainage, and signage components also had to be coordinated in real-time to keep all disciplines moving forward.

Another major challenge involved moving the excess dirt on the project site. The proposed plan including moving 275,000 cubic yards of imported borrow—the dirt that is delivered to help construct the site. To resolve the issues of the delay and removal of excess dirt from the project, our team coordinated with the city’s Department of Public Works to re-sequence the staging. This included closing one side of the over-crossing bridge as well as the westbound on-ramp and the eastbound off-ramp. While this created detours for commuters, the decision enabled us to combine three phases of the construction process into one. This resulted in several benefits:

• Reducing the offhaul of roadway excavation material
• Removing the inefficient balancing of dirt in each state of construction. We reduced the transported borrow quantity from 275,000 cubic yards to 225,000.
• Trimming four months from the project schedule
• Providing a roughly $350,000 credit to the city following a value engineering change proposal (VECP)

A VECP recommends a cost and/or time-saving measure that results in shared savings with the owner. In this case, the VECP, which ultimately yielded both cost savings as well as the four-month reduction in the schedule, was initiated by the contractor, reviewed by our CM team, and then cleared with the city’s risk and public relations departments. Ultimately, this effort mitigated much of the utility delay.

A Partnering Process

The successful process to address the challenges of the SR 120/Union Road interchange project in Manteca can be attributed in large part to a formal partnering relationship involving a professional facilitator. We maintained a strong relationship with the contractor, kept an open dialog with all parties, and encouraged collaboration and regular communications. This breakthrough project should soon serve as a model for other diverging diamond interchange projects in California.

Steve Kuntz, PE, DBIA, is a vice president in the Fairfax, Virginia, office of Dewberry.

Howard Zabell, PE, is an associate vice president and construction services manager in the Rancho Cordova, California, office of Dewberry.