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. 

Its circuitous one-way routing slowed traffic and has often led to congestion backing up onto I-5, as well as created conditions that contributed to over 200 accidents that have occurred here every year. Its vehicle-based design has been difficult for pedestrians and bicyclists to navigate. Finally, the public infrastructure there was obsolete and in poor condition, with pavement, sidewalks, and signals that needed replacement and some utilities along the corridor being over 100 years old.

While construction on Mercer East won’t be finished until mid-2013, Mercer Street opened to two-way traffic from Dexter Avenue N to I-5 on August 27.

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 Avenue N.

The Mercer West Project will complete the two-way Mercer Street connection between I-5 and Elliott Avenue W and connect neighborhoods to the SR 99 Tunnel by moving construction west of Dexter Avenue N.  The project, which will begin construction in early 2013, will widen Mercer Street between Dexter Avenue North and Fifth Avenue North for two-way traffic and modify signals and channelization to convert Mercer and Roy to two-way streets west of Fifth Avenue North.  Wider sidewalks at the SR 99 Underpass and a continuous bikeway from Dexter Avenue North to Queen Anne Avenue North are also part of Phase 2.

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 in the Underpass area include:

• Creating a new Sixth Avenue N connecting Mercer and Harrison streets to the SR 99 Tunnel;
• In conjunction with the Alaskan Way Viaduct Replacement Project, closing Broad Street to re-connect the street grid between Dexter Avenue N and Fifth Avenue N, reopening John, Thomas, and Harrison Streets across Aurora Avenue N;
• Rebuilding Thomas Street, Harrison Street, and Taylor Avenue North across the existing Broad Street right-of-way to complete the street grid;
• Building a bike path on the West side of Fifth Avenue North between Mercer and Republican streets that will be part of a future connection to Denny Way.
  
  
• Computer simulation #1, Mercer St. underpass for pedestrians
• Computer Simulation #2, Mercer St. underpass from roadway
• Typical roadway cross-sections diagram
• Public art diagram

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
• Sequoia at Mercer St. and Aurora Ave.: The existing Sequoia tree located in the triangle bounded by Mercer, Broad, and Aurora will be 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 will 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

• Two lanes in each direction on Mercer Street (Fifth Avenue N to Second Avenue N);
• One lane in each direction on Roy Street (Fifth Avenue N to Queen Anne Avenue N);
• Bike lanes on Roy Street;
• New signal on Mercer Street at Warren Avenue North;
• Additional marked crosswalks
• New and enhanced ADA ramps;
• Median island for pedestrians crossing Mercer at Fifth Avenue West.
• All changes occur within the existing right-of-way