SR 520 - I-5 to Lake Washington - Project design

Overall project vision – "Nature meets City"

Montlake lid rendering

The SR 520 corridor is a critical regional highway that is being rebuilt to support many modes of travel. The improved corridor in Seattle will:

  • Create infrastructure that embraces Seattle’s unique natural setting and promotes a vibrant urban environment.
  • Support regional and local connectivity and enhance the environment, with a special focus on shoreline habitat and the use of low-carbon materials.
  • Provide a positive experience for all users, including motorists, pedestrians, bicyclists, transit users, recreationists and residents of adjacent neighborhoods.


Key design elements

Here are three phases of construction ahead between I-5 and Lake Washington:

Montlake Phase

  • A new, seismically stronger West Approach Bridge South to carry three lanes of eastbound traffic past Montlake Boulevard to the new floating bridge.
  • A community-connecting highway lid and improved interchange at Montlake Boulevard.
  • A bicycle-pedestrian land bridge over SR 520 east of the lid.

Portage Bay Phase

  • A seismically stronger Portage Bay Bridge to replace the existing bridge, which is vulnerable to earthquakes.
  • A community-connecting lid at 10th Avenue East and Delmar Drive East.
  • A landscaped, 30-foot-wide bicycle and pedestrian shared-use crossing over I-5.

Montlake Cut Crossing Phase

  • A new, second bascule bridge across the Montlake Cut to provide additional capacity and safer north-south travel on Montlake Boulevard.

Additional design resources:

Public engagement

WSDOT has worked since the late 1990s with the city of Seattle, agency partners, stakeholders and the public to develop, analyze and refine SR 520 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 a not-building-anything option. Analysis and public feedback led WSDOT to drop the four-lane alternative and study further the no-build and six-lane alternatives. In 2010, following additional public review and feedback, a six-lane "Preferred Alternative" design was endorsed.

The Preferred Alternative went through more 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.

From 2014 through 2016, a team of design professionals, working in collaboration with the Seattle Design Commission and supported by WSDOT and city staff, recommended design refinements. Check out our final concept design report for more details.

Visit our Montlake Phase page to learn about ways to stay engaged as we prepare for construction.