SR 520 Montlake Project

SR 520 Montlake Project

Seattle, Washington
United States
SR 520 Montlake Bridge - Flickr WSDOT

TYLin is the lead engineering firm for the State Route 520 Montlake Project, a design-build endeavor in Seattle, Washington.

TYLin is part of the Graham Contracting, Ltd. team selected by the Washington State Department of Transportation to improve transportation for both motorized and nonmotorized travel along State Route (SR) 520. As Lead Engineer, TYLin is providing design management and serves as Engineer of Record for all structural components.

The Montlake Lid Bridge takes center stage and presents enormous challenges as it is being constructed over an active freeway and the Montlake Interchange. To maintain daily traffic of close to 60,000 vehicles on SR 520, the three piers of the lid bridge substructure were built in nine segments. The last remaining segment was completed in April 2023. 

The project includes numerous unique structural, community, and transit elements. The new lid structure accommodates local roads, high-occupancy vehicle (HOV) facilities, sidewalks, and bus stops. The project also includes a separate signature bicycle and pedestrian land bridge, ride shared-use path tunnel, and an over-water viaduct approach consisting of 41 spans of base isolated prestressed girders (1.1 miles) to replace the final segment of the seismically vulnerable SR 520 bridge. 

During the design process, the team developed presentation content focused on stakeholder interests. We facilitated design workshops with the Seattle Department of Transportation and King County Metro to detail value-added design specifics and gain buy-in on both the overall approach and detailed design elements. 

TYLin devised an innovative construction strategy to minimize impacts on the community and mobility. It requires only five significant SR 520 mainline traffic shifts, which assures driver reliability, safety, and operation optimization. Our strategy also allows the project to be completed quickly with fewer night, short, and full weekend closures of SR 520 and local streets. This approach also protects the historic Section 106 properties in the Montlake neighborhood and significantly reduces noise, dust, and vibration impacts.

Project Highlights:

  • Construction of the remaining lid elements over SR 520 continues while traffic on Montlake Boulevard is maintained, routing the roadway and sidewalks over the completed lid segments. 
  • The lid also features a 3-acre aesthetic park element and community space to enhance neighborhood connectivity and reduce traffic noise impacts. 
  • The project restores a regional trail system connection between the arboretum south of SR 520 and the neighborhood to the north – the original connection was severed when SR 520 was constructed in the 1960s. 
  • This project involved many stakeholders including Washington Park Arboretum, Seattle Public Utilities, and Seattle City Light. 
  • Construction of the Montlake Project is the first of four SR 520 projects that make up the "Rest of the West" improvements.
  • The full corridor enhancements will feature stronger bridge construction, bus/carpool lanes and a regional bicycle/pedestrian path running from Seattle to Redmond, safer on- and off-ramps, and improved connections to transit, local streets, and trail networks. 

Images Credit: Washington State Dept. of Transportation Flickr