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WSDOT Develops First Diverging Diamond Interchange in Lacey

The image shows a graphic overlay of WSDOT’s new diverging diamond interchange on Marvin Road and Interstate 5 in Lacey, Washington.

On 22 August 2018, construction bidding started for Washington State’s first diverging diamond interchange (DDI) for Washington State Department of Transportation (WSDOT) for Interstate 5 (I-5) in Lacey. Although it’s the first one built in Washington State, over 70 versions have been built throughout the United States. The interchange will be located between I-5 and State Route 510, where the new crisscrossing lanes will be built—updating ramp terminals, widening the existing bridge, and adding a southbound lane.

Currently, additional pressure was added to the interchange because of the increase of traffic, especially northbound, from the widening of Marvin Road and rebuilding of Hogum Bay Road. The potential benefits of the DDI are not just the improvement of the flow of traffic, but the increase in overall safety, allowing opportunities to develop a multimodal facility, moving a higher capacity of vehicles, and creating space for economic development. Expanding on traffic improvements, WSDOT’s style of DDI is designed to coincide with the nearby intersections that improve supporting arterial flow; this will reduce overall traffic backups and allow on- and off-ramps to be updated, the addition of a southbound terminal, the widening of the existing bridge, and addition of dedicated bike lanes and sidewalks.

The DDI works by shifting two lanes of westbound traffic from the right side of the road to the left side and shifting eastbound traffic from the right side to left side of the road. The DDI will be built on the non-freeway road, Marvin Road; traffic will be taken on both opposite sides of the bridge at the freeway on- and off-ramps. Less time will be spent waiting for traffic signals, which will increase traffic efficiency. Supporting future population growth in the area, DDIs move more people in high-traffic areas and increase safety. Decreasing traffic buildup and increasing the ease of moving a higher number of people through the Lacey/I-5 corridor area benefits local brick-and-mortar businesses, thereby stimulating economic activity in the area. A multimodal design, WSDOT’s DDI is a bicycle- and pedestrian-friendly design with protected barriers that reduce the risk of bike-pedestrian-vehicle collisions. The design eliminates left turns, allowing traffic to keep flowing by decreasing potential opportunities for conflicts and collisions.

For this effort, WSDOT contracted BergerABAM to accomplish a wide range of tasks from construction staging, landscaping, public involvement, creating a Transportation Management Plan, to construction scheduling. During the preconstruction phase, BergerABAM will be responsible for the traffic modeling, finalizing the project scope, preparing interchange plans for approval, completing the hydraulic report, developing construction staging strategy and plans, determining right-of-way limits, and preparing all civil plan sheets and special provisions. During the construction phase, BergerABAM’s responsibilities will include providing support to the WSDOT construction project engineer to address constructability issues and responding to questions related to design and plan sheets.

When introducing a new transportation initiative, stakeholder engagement is critical to the adoption of the DDI and ensuring the success of local businesses. From project ideation through construction and fruition, BergerABAM’s public involvement specialists developed and implemented a public outreach program to educate the public on the benefits of the DDI. The communication plan includes business and stakeholder meetings and public open houses, which gave the public an opportunity to provide WSDOT feedback and for WSDOT to share objective information looking to optimize the ability for communities to thrive during construction.

Construction is projected to begin towards the end of 2018.