SR 520 - I-5 Express Lanes Connection Project

Project news

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 SR 520 / I-5 Express Lanes Connection Project is one of the major stages of SR 520 reconstruction for the highway's west side corridor.

Needs & Benefits

Seattle is experiencing a population and employment boom, especially in the South Lake Union neighborhood. At the same time, the Eastside’s technology-driven cities continue to experience rapid growth. The region’s growth underscores the importance of a safe, reliable transit/HOV link between Seattle and the Eastside via SR 520. This project will extend SR 520’s new transit/HOV system onto the I-5 express lanes, creating a direct bus and carpool connection between SR 520, South Lake Union and downtown Seattle.

The end result

  • A new, reversible transit/HOV ramp between SR 520 and the I-5 express lanes
  • A modified, reversible ramp between the I-5 express lanes and Mercer Street
  • Restriped I-5 express lanes to retain the existing four general-purpose lanes while adding a transit/HOV lane

graphic showing SR 520 / I-5 express lanes connection

Watch a brief video animation showing how the reversible transit/HOV connection will operate.

Project benefits

  • Safer, more reliable travel between the Eastside’s growing cities and Seattle’s booming South Lake Union neighborhood from a new, reversible transit/HOV lane connecting SR 520 and Seattle’s I-5 / Mercer Street interchange
  • Earlier-than-planned Eastside / South Lake Union transit service via the reversible I-5 express lanes connection; the connection will open as early as 2023 for transit instead of the originally expected opening in 2029
  • A more efficient phasing of SR 520 construction that minimizes contractor overlap in the Montlake area while keeping regional and local traffic moving during construction


  • 2020: begin construction (estimated)
  • 2023: complete construction* (estimated)

* Note: The reversible transit/HOV lane initially will open only to transit; carpools will use the reversible lane later, following completion of the new Portage Bay Bridge and Roanoke lid.


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.

thumbnail map of SR520/I-5 Express Lanes connection