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Mercer Corridor Project Overview
Mercer West Dexter to 5th Avenue W
Project Background
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Mercer Corridor: Project Background

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, along with growing numbers of pedestrians and bicyclists, 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. opened to two-way traffic between I-5 and 9th Ave. N in August 2012. Since then, construction has continued on nearby Fairview Ave. N and Valley St. The first phase will be completed in fall, 2013.

The next phase of construction, between Dexter Ave. N and 5th Ave. W, began in early spring 2013 and is anticipated to be complete by mid-2015. When complete, the Mercer Corridor will carry two-way traffic between I-5 5th Ave. W and connect Seattle neighborhoods to the north portal of the SR 99 Tunnel.

click to view larger

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.

The Problem

Its circuitous one-way routing slowed traffic, often led to congestion backing up onto I-5 and created conditions that contributed to over 200 accidents 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.

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 Avenue 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.

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:

  • Creating a new 6th Ave. N connecting Mercer and Harrison streets to the SR 99 Tunnel
  • In conjunction with the Alaskan Way Viaduct Replacement Project, closing Broad St. to re-connect the street grid between Dexter Ave. N and Fifth Ave. N, reopening John, Thomas, and Harrison streets across Aurora Ave. N
  • Rebuilding Thomas St., Harrison St., and Taylor Ave. N  across the existing Broad St. right-of-way to complete the street grid
  • Building a bike path on the West side of 5th Ave. N between Mercer and Republican streets that will be part of a future connection to Denny Way

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

click to view larger

  • 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
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