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SR 520 - I-5 to Lake Washington Project Design

Overall project vision – "Nature meets City"

As envisioned by WSDOT, the city of Seattle and other community interests, the SR 520 corridor is a critical regional highway that will support many modes of travel. WSDOT has the opportunity to build a more vibrant transportation facility through Seattle that could:

Montlake lid rendering
  • Build upon Seattle’s vision set forth 100 years ago to become a premier city by creating infrastructure that embraces Seattle’s unique natural setting and promotes a vibrant urban environment.
  • Yield practical solutions and sustainability practices that support regional and local connectivity as well as the environment, with a special focus on shoreline habitat and the use of low-carbon materials.
  • Provide a memorable experience for all users, including motorists, pedestrians, bicyclists, transit users, recreationists and residents of adjacent neighborhoods.

View an animated video of the project conceptual design on our YouTube page.

Key design elements

The planned and fully funded upgrades between I-5 and Lake Washington include:

  • A seismically stronger Portage Bay Bridge to replace the existing bridge, which is vulnerable to earthquakes.
  • Community-connecting highway lids at Montlake Boulevard and 10th Avenue East / Delmar Drive East.
  • A new, seismically stronger West Approach Bridge South to carry three lanes of eastbound traffic past Montlake Boulevard to the new floating bridge. This is the complementary bridge to the south of the West Approach Bridge North, which is under construction and set to open in 2017.
  • A new, second bascule bridge across the Montlake Cut to provide additional capacity and safer travel.
  • A 30-foot-wide, landscaped bicycle and pedestrian shared-use path over I-5.
  • Related mitigation projects, including aquatic mitigation at Taylor Creek and Seward Park, aquatic and wetland mitigation at Magnuson Park, landscaping enhancements on Foster Island, a new Arboretum north entry, trail improvements in the Portage Bay area, community construction management plans for future construction phases, historic documentation and interpretive signage. (More information is available on our Environmental Stewardship page.)

Check out our report showcasing the final concept design, published in February 2016. The report contains a wide variety of renderings, designs and diagrams of the planned improvements.

Public engagement

Throughout the life of the SR 520 reconstruction project, WSDOT has worked with agency partners, stakeholders and the public to develop, analyze and refine designs and solutions.

In 2006, the draft environmental impact statement considered three choices for the project: a four-lane highway, a six-lane highway, and not building anything. Analysis and public feedback led WSDOT to drop the four-lane alternative and study further the no-build and six-lane alternatives. These alternatives were developed and subjected to additional scrutiny and public feedback over the next several years. One milestone during this process was the creation of a six-lane "Preferred Alternative" design in 2010.

This design then went through additional rounds of analysis and refinement, most notably the Seattle Community Design Process in 2011-2012. This robust and collaborative effort between WSDOT, the city of Seattle, design professionals, and the broader public resulted in a refined corridor vision and conceptual design. While many well-supported design refinements were endorsed and incorporated into the preliminary concept design, the city asked WSDOT to continue exploring the design of the Portage Bay Bridge, Montlake lid, and nonmotorized connectivity.

In 2014, a team of design professionals, working in collaboration with the Seattle Design Commission and supported by WSDOT and city staff, recommended design refinements.