Mercer Corridor Project

For more than 40 years, the Mercer Corridor has been one of the City’s most significant transportation challenges. Some 80,000 vehicles at the I-5 interchange travel the Mercer Corridor between Elliott Ave. W and I-5 each day, making it a critical east/west route for keeping people, goods, and services moving.

In early 2010, SDOT began the first phase of construction between I-5 and Dexter Ave. N and Mercer St. and opened to two-way traffic between I-5 and 9th Ave. N in August 2012. Construction continued on nearby Fairview Ave. N and Valley St. The first phase was completed in fall 2013.

The next phase of construction, between Dexter Ave. N and 5th Ave. W, began in early spring 2013 and is was complete by 2015. The Mercer Corridor now carries two-way traffic between I-5 and 5th Ave. W and connects Seattle neighborhoods to the north portal of the SR 99 Tunnel.

Map of Mercer Corridor

A two-way Mercer Corridor improves mobility for all modes of transportation:

  • Creates an efficient and direct east/west transportation corridor between I-5, SR 99 and 5th Ave.
  • Reduces conflicts between cars, trucks, pedestrians and bicyclists
  • Improves pedestrian and bicycle safety and access
  • Strengthens connections among area neighborhoods
  • Improves access to and from Seattle Center
  • Accommodates and encourages future transit investments

Mercer East (Dexter Ave N to I-5)

The "Mercer Mess" has been a major bottleneck in the Seattle for decades, and has been hindering access to Seattle's fastest growing neighborhood, South Lake Union.  The old Mercer and Valley streets couplet was constructed in the late 1950s as a temporary solution intended to provide access to I-5 as it was being built.

The previous configuration of Mercer and Valley streets is shown below.

Mercer Corridor map

Its circuitous one-way routing slowed traffic, often led to congestion backing up onto I-5 and created conditions that contributed to over 200 crashes or collisions that occurred nearby each year. The vehicle-based design was difficult for pedestrians and bicyclists to navigate and the public infrastructure was obsolete and in poor condition. The pavement, sidewalks, and signals need replacement as some utilities along the corridor are over 100 years old.

While construction on Mercer East won’t be finished until fall 2013, Mercer St. opened to two-way traffic from Dexter Ave. N to I-5 on August 27, 2012.

Mercer Corridor map

There are now three eastbound lanes traveling onto I-5 (the same number of through lanes to I-5 that Mercer previously had), three westbound lanes (currently operating from I-5 to 9th Avenue N), plus a far right through lane for vehicles traveling to Capitol Hill (as previously existed), and a new left-turn lane onto Fairview Ave. N.

Mercer West (Dexter Ave N to 5th Ave W)

The Mercer West Project will complete the two-way Mercer Street connection between I-5 and 5th Ave. W and connect neighborhoods to the SR 99 Tunnel by moving construction west of Dexter Ave. N. The project, which began construction in early 2013, will widen Mercer St. between Dexter Ave. N and 5th Ave. N for two-way traffic and modify signals and channelization to convert Mercer and Roy to two-way streets west of 5th Ave. N. Wider sidewalks at the SR 99 Underpass and a continuous bikeway from Dexter Ave. N to Queen Anne Ave. N are also part of the West Phase.

Mercer Corridor map
Widened Mercer Underpass

The widened Mercer Underpass will provide three lanes of traffic in each direction, left turn lanes, wider sidewalks (considerably wider than the current 5’ sidewalk width) and a bike path.

Additional project benefits of the underpass area include:

Tree Additions and Removals

Mercer West will plant 2 new trees for every tree removed during the project.

  • Totals: 53 trees to be removed, 110 to be planted, 1 relocated
  • Most new trees to be planted along Mercer St. between 5th and 9th Avenues N
  • Will use ‘Silva Cells’ system to better ensure growth and prevent root damage to sidewalks
  • The existing Sequoia tree located in the triangle bounded by Mercer St, Broad St, and Aurora Ave. N has been removed during Mercer West. Planted in 1958, SDOT considered relocating the tree but ultimately could not due to cost and space restrictions. To mitigate the loss of the Sequoia, SDOT will plant two 20-foot Sequoia trees with the guidance of SDOT’s Urban Forestry and Landscape Architect and support Urban Forestry’s efforts to improve growing conditions for the historic tree at Stewart St. and 4th Ave. N.
  • Tree addition/removal diagram

Converting Mercer and Roy streets to two-way operation

Mercer Corridor staging map

  • Two lanes in each direction on Mercer St. (5th Ave. N to 2nd Ave. N);One lane in each direction on Roy St. (5th Ave. N to Queen Ave. N);
  • Bike lanes on Roy St.;
  • New signal on Mercer St. at Warren Ave. N;
  • Additional marked crosswalks
  • New and enhanced ADA ramps;
  • Median island for pedestrians crossing Mercer St. at 5th Ave. W.
  • All changes occur within the existing right-of-way


Mercer West Major Milestones

November 2014: Opened third eastbound lane on Mercer St. between 5th Ave. N and 9th Ave. N, and opened two new left turn lanes from westbound Mercer St. onto southbound 5th Ave. N.

June 2014: Opened Mercer St. between 1st Ave. N and 9th Ave. N; Roy St. between Queen Anne Ave. N and 5th Ave. N; and Queen Anne Ave. N between Roy and Mercer streets to two-way traffic. Closed Broad St. between 9th Ave. N and 5th Ave. N to accommodate the widening of Mercer St.

December 2013: Completed demolition of the western half of the SR99 bridge over Mercer St.

September 2013: Fully re-opened Westlake Ave. N, Valley St. 9th Ave. N and Broad St. to traffic, marking the end of major roadway closures associated with the East Phase of the Mercer Corridor Project.

May 2013: Completed demolition of the eastern half of the SR99 bridge over Mercer St.

April 2013: Construction began on Mercer West, between Dexter Ave. N and 5th Ave. W. SDOT and WSDOT created a construction schedule and map showing the timing and interaction of construction for the Mercer Corridor project and the North Portal of the SR 99 Tunnel.

March 2013: Fully re-opened traffic on Fairview Ave. N, transitioned to Stage 4 of Mercer East construction

August 2012: Opened Mercer St. to two-way traffic between I-5 and Dexter Ave. N., transitioned to Stage 3 of Mercer East construction

January 2012: Completed new lanes of Mercer St. and switched traffic, transitioned to Stage 2 of Mercer East construction

June 2011: 9th Ave. N converted to underground power, new gas line installed

May 2011: Completed replacement of 9th Ave. N sewer and associated side sewer

 September 2010: Groundbreaking ceremony held at Mercer St. and Westlake Ave. N

July 2010: Completed building demolition and construction site prep north of Mercer St.

June 2010: Awarded construction contract to local contractor Gary Merlino Construction Company

March 2010: Began building demolition and construction site preparation north of Mercer St.

February 2010: Received a $30 million federal Transportation Investments Generating Economic Recovery (TIGER) Grant


Greg Spotts, Director
Address: 700 5th Ave, Suite 3800, Seattle, WA, 98104
Mailing Address: PO Box 34996, Seattle, WA, 98124-4996
Phone: (206) 684-7623

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The Seattle Department of Transportation (SDOT) is on a mission to deliver a transportation system that provides safe and affordable access to places and opportunities for everyone as we work to achieve our vision of Seattle as a thriving, equitable community powered by dependable transportation.