In 2017, Sound Transit and the Washington State Department of Transportation (WSDOT) began creating a plan to improve transit access and active transportation connectivity at the I-405 and Northeast 85th Street interchange in Kirkland, Washington. The plan includes a Bus Rapid Transit (BRT) station that supports Sound Transit’s Bus Rapid Transit S2 line and is one piece of the future High-Capacity Transit system.
“This project is the result of years of interagency planning and coordination among the public, Sound Transit, the City of Kirkland, King County Metro, and WSDOT,” said David D. Gerla, a Design Manager on the WSDOT I-405/SR 167 Program.
After years of planning and many design concepts, Sound Transit, WSDOT, and project stakeholders created a plan that balances efficient vehicular, transit, pedestrian, and cyclist traffic. The design calls for constructing a new, multi-level, multiple-roundabout interchange with BRT stations, Express Toll Lanes, direct access, and pedestrian and bike connections.
WSDOT is also actively removing culverts that were found to hinder salmon migration, as well as replacing a 30-inch corrugated metal pipe that carries a tributary to Everest Creek under I-405 with a 14-foot-wide culvert that provides space to flow naturally.
Completing the intricate project while keeping traffic moving on Northeast 85th Street and I-405 required complex and meticulous planning and sequencing.
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“We put a huge emphasis on a construction staging plan,” Gerla said. “It's one thing to come up with a design that works, but it's another to come up with a construction plan. We spent a year putting together a preliminary construction staging plan, but it took several years to develop overall."
The final plan is a $234.4 million design-build project taken on by Alberta, Canada-based contractor Graham Construction & Engineering and designer Parsons Corporation. The project turns the existing two-level intersection into a three-level interchange.
“Figuring out how to build a three-level interchange within an existing interchange was a challenge,” Gerla said. “We used modern design and traffic analysis tools to test concepts early on in an effort to vet construction staging and feasibility of the plan. We made sure we could illustrate and communicate how concepts would work to gain support from all agencies early on in the planning process.”
Each level is designed with specific goals in mind: to keep local traffic flowing safely, to provide safe access to public transit and active transportation, and to allow for easy access to the interstate. Here is a breakdown of the construction planned on all three levels.
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The bottom level consists of the Northeast 85th Street and I-405 general-purpose ramp intersection. To accommodate the new level, the design called for lowering the local road approximately 25 feet, requiring more than 300,000 yards of dirt to be excavated.
The team constructed a temporary diamond interchange, new I-405 mainline, and Northeast 85th Street bridges with foundations outside the area for excavation. Then, the team switched traffic to the new structures and demolished existing I-405 loop ramps and bridges to begin the excavation. A 24-inch City of Kirkland water main and Puget Sound Energy gas line that ran under the existing roadway were relocated to accommodate the excavation.
The team constructed soldier pile walls to support the excavated area and hold the future bridge foundations and roadway above level two. To complete level one, the team created an oblong-shaped, dual teardrop roundabout for traffic moving through the interchange between Northeast 85th Street and the I-405 general-purpose lanes.
The middle level is a low-speed, low-volume environment that will be used by Sound Transit, King County Metro, Express Toll Lane users, and active transportation like pedestrians and cyclists.
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The team needed to demolish existing mainline bridges, foundations, pavements, and retaining walls in the freeway median. The level features six single-span concrete bridges connecting Northeast 85th Street with the I-405 Express Toll Lanes at a new roundabout. Two of the bridges run north to south, crossing over level one, and four run east to west, above the I-405 general-purpose ramps. The bridges and approach roadway are supported behind new soldier pile walls.
Level two will include four new bus rapid transit stops. Two are Sound Transit Stride stations, and two are King County Metro future RapidRide stops. All transit platforms will be 15 feet wide.
“The interchange creates a transit center where stations for Stride and future RapidRide service are on the same level and in close proximity, with no vertical transfer,” Gerla said. “A transfer between buses will require only a short, at-grade walk.”
The design includes a drop-off/pick-up facility for transit users and a connection to a nearby neighborhood, with plans for three additional neighborhood connections in the future, should funding be identified. Pedestrians will use new sidewalks that are 12 feet wide on average for walkable access to the transportation hub.
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Level two also includes direct access ramps for drivers using the express toll lanes on I-405.
“In the former freeway configuration, Express Toll Lane access required buses and passenger vehicles to weave across the general-purpose lanes. Weaving is a challenge for transit and a contributor to congestion,” Gerla said. “By reducing weaving maneuvers and providing Express Toll Lanes direct access, we are working toward achieving the broader goals of reducing congestion and providing safe, reliable access to alternative modes of transportation.”
All of these features are located in another oblong-shaped roundabout. WSDOT worked with Sound Transit and King County Metro well before construction began to ensure the roundabout’s shape would work with bus drivers, a potential point of contention. While roundabouts are effective at traffic control, they can be difficult to navigate for bus drivers if not considered during design.
“There are competing objectives for roundabouts,” Gerla said. “They typically are designed narrowly to encourage slower speeds, but that's not bus-friendly.”
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In the design phase, WSDOT and Graham Construction & Engineering demonstrated that the bus models expected to travel through the interchange could safely navigate the roundabouts. They worked with Sound Transit and King County Metro to test the roundabout design in the field, prior to construction. The transit agencies staked out the future design — including the roundabout and station platform positions — in a large parking lot and invited bus drivers to drive through the design with all bus types. The design passed the test, ensuring the roundabouts would work for passenger vehicles and buses.
On the top level, I-405 will be approximately the same elevation as the former I-405 overpass, thanks to the decision to lower level one by 25 feet. To create the top level, the team demolished existing bridges, foundations, and walls. They built two 300-foot-long, 80-foot-wide, two-span concrete bridges. Each span is between 142 feet and 158 feet and has seven to eight 74-inch-deep concrete girders. The bridges are supported by six columns on drilled shafts that are 8 feet in diameter.
The center piers for the bridges are located in the median of Northeast 85th Street on level one, and they were constructed before level one was excavated 25 feet. Constructing the center pier prior to excavation required a unique strategy. The team leveraged 12-foot-diameter temporary construction casings to excavate the area of the pier columns 25 feet below existing ground, and then they installed the drilled shafts an additional 70 feet deeper. As the team excavated down to level one, the casings were removed to expose the lower parts of the columns.
“This kind of technique has been done before, but it’s not common,” Gerla said.
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After years of planning, communication, collaboration, and problem-solving, the project is expected to streamline traffic and improve transit access in Kirkland. WSDOT has met weekly and biweekly with Graham Construction & Engineering and all agency representatives. Two projects are occurring adjacent to the interchange, so the team meets regularly with those representatives as well.
“This project involved all stakeholders coming together, as well as the public. We held open houses and went to neighborhood meetings to show the public what we were planning and invited their comments and feedback,” Gerla said. “A lot of that work shaped the final concept of what the project became.”
The I-405/Northeast 85th Street interchange and inline BRT station project is scheduled to wrap up in early 2027, with substantial completion scheduled for November 2026.
The I-405/Northeast 85th Street interchange and inline BRT station project is one component of two master plans in the area.
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- The Sound Transit Regional Transit System Plan calls for new bus rapid transit stops for the agency’s Stride S2 Line, which runs from Bellevue to Lynnwood, Washington.
- WSDOT’s I-405 Master Plan includes more than 150 projects to improve travel between Lynnwood and the Renton/Tukwila area in Washington, according to WSDOT. The project is providing direct access to 85th Street via express toll lanes as part of that plan.
- 350,000 cubic yards of excavation
- 80,000 tons of hot mix asphalt
- 55,000 square feet of new sidewalk
- 22 acres of flow control drainage
- 14 acres of stormwater treatment
- 60,000 square feet of new cutwall
- 15,000 square feet of fill wall
- 10,000 square feet of noise wall
- 13 overhead sign structures
- Five toll gantries
- Eight new bridges
- Owner: Washington State Department of Transportation, Olympia, Washington; Sound Transit, Seattle, Washington
- Contractor: Graham Construction & Engineering, Calgary, Alberta, Canada
- Designer/Engineer: Parsons Corporation, Chantilly, Virginia
- Agencies: City of Kirkland, Washington; King County Metro, Seattle, Washington Photos courtesy of WSDOT
