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Mercer Corridor Project – The Importance of Improving Infrastructure

The Mercer Corridor Project West Phase is progressing toward completion later this year. After a few years of construction in the Mercer Corridor, with associated restrictions in mobility for all modes, Mercer Street, with new lanes, concrete pavement and sidewalks, and separated bike lanes will be fully operational.

West Mercer Update

Mercer Corridor Aerial Concept

The Mercer Corridor links I-5 to Elliot Ave. W and is one of the most critical east/west routes in the city serving tens of thousands of travelers and freight haulers every day in Seattle’s fastest growing neighborhood. Prior to construction, the Mercer Corridor was a one-way eastbound arterial (Mercer St.) with an indirect westbound route that caused delays and created conflicts between vehicles, pedestrians and bicyclists. In addition, the Mercer Corridor had an inadequate pedestrian environment, with narrow and aging sidewalks, no separate bicycle lanes, and obsolete traffic signals. Each of these issues was addressed as part of the Mercer Corridor Project design. The Mercer West Project will:

Widen Mercer Street to create a two-way arterial with three lanes in each direction across SR99:

  • Affects 37,000 daily drivers heading west from I-5
  • Reduces vehicle miles traveled (VMT) which leads to reduction in Greenhouse Gas Emissions
  • Provides direct truck connections between I-5 and the Ballard-Interbay Manufacturing and Industrial Center.
  • Provides a connection between the SR 99 Bored Tunnel and neighborhoods west of SR 99.
  • Reduces conflicts between vehicles, bicycles, and pedestrians.
Mercer Street facing East

Mercer Street facing East

Upgrade signals

  • Will adapt to changing traffic conditions
  • More energy efficient
Mercer Street at 5th Ave

Mercer Street at 5th Ave

Improve pedestrian mobility and access

  • Creates safe convenient crossings at intersections on Mercer and Roy streets, and brings crossings into compliance with the Americans with Disabilities Act.
  • Widens sidewalks on Mercer St. across SR99
  • Removes major conflict points between turning traffic and pedestrians.


Improve bicycle mobility and access

  • Creates a continuous bicycle connection from Fairview Ave N to Queen Anne Ave N with bicycle lanes on Valley and Roy streets and a separated pathway on Mercer across SR 99.
  • Provides the first block of future separated bike lanes on Fifth Ave N between Mercer and Denny Way.


Replace aging infrastructure

  • New concrete pavement on Mercer St.
  • New Sixth Ave N connecting Mercer to Harrison St, as well as the SR 99 Tunnel.
  • Replaces the SR99 bridge over Mercer St, bringing it to current seismic standards.
  • Reinforces the retaining wall on the north side of Mercer St.
  • Installs energy-efficient LED street lights
  • Replaces 80-115 year old water and sewer mains
  • Installs new stormwater detention facilities and treatment facilities to protect Lake Union
  • Undergrounds the 115 kV Broad-University Transmission Line
  • Installs Distribution System Capacity Enhancements for Seattle City Light’s customers


Although construction has been lengthy, the Mercer Corridor Project is an investment in our economy and our future. The Mercer Corridor Project improvements will support the 38,000 new jobs and 18,000 new households expected in the area by 2024.

The projected benefit of the Mercer West Project attributed to the investment in new infrastructure is estimated to be $100 M to $350 M over the next 25 years. The cost-benefit analysis is summarized in the Federal Highway Administration TIGER IV Grant Application, which is located on the project website:

SDOT and our crews look forward to the progress and completion of construction in 2015 and thank you for your patience during construction.

For up-to-minute construction updates join our project email list at: or call the 24-hour construction hotline at 206-419-5818.