SR 520 - Portage Bay Bridge and Roanoke Lid Project

We are preparing to move forward, in phases, with construction of the remaining improvements to SR 520 in Seattle – what we call the “Rest of the West.” The Portage Bay Bridge and Roanoke Lid Project is one of the major phases in the SR 520 Bridge Replacement and HOV Program's reconstruction of the highway's west side corridor.

Project news

  • For the past four months we've been soliciting public feedback on design features for the Portage Bay Bridge and Roanoke Lid Project. We started with a June open house (pdf 1.6 mb), followed by an online open house and a series of Community Stakeholder Workshops. We're holding another in-person open house on Oct. 29 to share the latest updates on the Portage Bay project and our three other Rest of the West projects. Join us from 5:30 to 7:30 p.m. Tuesday, Oct. 29, at the Montlake Community Center, 1618 E. Calhoun St., in Seattle. 

Needs & Benefits

The 1960s-era Portage Bay Bridge is nearing the end of its functional life. Supported by hollow concrete columns, the four-lane bridge could fail in a severe earthquake. This project will replace the old bridge with a seismically stronger structure. The project also will extend SR 520’s transit/HOV system and cross-lake bicycle and pedestrian trail from Montlake to I-5.

The end result

  • Two new parallel, three-lane bridges across Portage Bay – one for eastbound traffic, the other for westbound traffic
  • Dedicated transit/HOV lanes across Portage Bay between Montlake and I-5
  • Extension from Montlake to I-5 of the regional SR 520 Trail
  • A landscaped Roanoke lid over SR 520, between 10th Avenue East and Delmar Drive East
  • A 30-foot-wide bicycle and pedestrian crossing over I-5
  • A system to capture and filter highway runoff before it enters local waterways

Project benefits

  • Safer travel from replacement of the old, structurally vulnerable Portage Bay Bridge with seismically stronger structures 
  • Improved regional mobility with completion of the final stretch of dedicated transit/HOV lanes, in each direction, between Redmond and I-5
  • Greater transportation options by completing the regional, cross-lake bicycle/pedestrian path along the entire SR 520 corridor, and a new, safer shared-use crossing over I-5
  • A community-connecting highway lid with landscaped open space
  • Improved water quality from a new system for treating SR 520 stormwater runoff
  • Fewer traffic backups thanks to wider highway shoulders that allow disabled vehicles to pull over without blocking traffic


  • 2023: begin construction (estimated)
  • 2029: complete construction (estimated)

conceptual rendering of a new Portage Bay Bridge

Conceptual rendering of a new Portage Bay Bridge.

Project design

Our website’s planning-documents webpage contains extensive information about the Seattle Community Design Process and other project-design initiatives and reports. There will be additional design work for future Rest of the West projects, with additional opportunities for public comment.


This project is part of the remaining $1.6 billion in SR 520 improvements from Lake Washington to I-5 – the “Rest of the West.” These improvements are funded by the 2015 Connecting Washington transportation legislation. For more information on SR 520 funding, visit the Budget and Performance page.


Here are some resources you can use to stay informed about this project:

If your organization is interested in receiving a project briefing or presentation, please contact us.