HBB Landscape Architecture revitalized seven blocks of the Mercer Corridor in the heart of Seattle, including Mercer Street (pictured), Valley Street, and Westlake Avenue North, to promote multimodal circulation. The corridor is designed to highlight the role of transportation in connecting public open spaces with neighborhoods, through the use of sustainable landscape techniques. HBB coordinated with public artist Ellen Sollod to provide earthwork and plantings in Mercer Street's median to complement the column sculpture "Origami Tessellation," by Sollod Studio. Some of the plantings include Japanese blood grass, Karl Foerster feather reed grass, and pink pavement rugosa roses.

As one of the main entries into the heart of Seattle, the Mercer Corridor functions as a gateway and vital east-west link between the Seattle Center/Space Needle, several Seattle neighborhoods encompassing theater and art districts, technology and biotechnology hubs, and two regional transportation corridors, Interstate-5 and State Route 99.

The Mercer Corridor East project is celebrated as a completed and built phase of the city's larger multimodal transportation vision. The Mercer Corridor East project converted a one-way Mercer St./Valley St. couplet into a traditional two-way street grid just north of Seattle's downtown core. Focusing on Mercer Street, Valley Street, and Westlake Avenue North, the new grid promotes strong multi-modal circulation and connections that didn't previously exist in the original automobile focused configuration. The new design highlights the role of transportation corridors in connecting public open spaces and neighborhoods, and the use of sustainable landscape techniques that address stormwater management, flood/drought resiliency, and multiple modes of movement.

The 20' wide median on Mercer Street treats stormwater run-off through a cascading series of detention areas planted with a variety of grasses and rushes, and provides space for large, boulevard London plane trees such as the one located behind the sculpture. Over 200 trees were planted for the project, helping meet some of Seattle's urban forest canopy goals. Several breaks located throughout the median provide generous space for pedestrian and bicyclist pause points at street crossings.

The design of the seven blocks of streetscape provides a gateway entrance from Interstate 5 and provides a north-south connection from the rapidly developing business and residential areas to Lake Union Park, the Museum of History and Industry, and additional waterfront associated recreation areas and services. Known to Seattleites for years as "The Mercer Mess," the project is part of a larger citywide plan to integrate complete street design principles with transportation infrastructure projects. This plan was envisioned and conceptually illustrated by a multidisciplinary team of engineers, urban designers, and landscape architects more than a decade prior to the start of construction.

Receiving partial funding from a federally sponsored TIGER grant (Transportation Investment Generating Economic Recovery), the project implemented complete street principles to improve access, safety, and circulation for increasing numbers of bicycle commuters and pedestrians, several Metro Transit bus routes, two Port of Seattle freight routes, a surface-line streetcar route, and vehicular traffic.

Mercer Street's "green fingers," landscaped bulb-outs along the street edges, protrude into the corridor to help delineate street-side parking and to calm traffic. The 16 green fingers are located at intersection crossings and rain garden medians. They are planted with sword ferns, elm trees, and golden Japanese forest grass.

 

The main theme, movement of people using a variety of transportation modes, is expressed in the landscape through the use of fine textured plants, especially ornamental grasses that visually move in the wind and wave as vehicles, buses, and streetcars pass by.

Several sustainability goals were met through the incorporation of large planting areas, increased planting soil depths, structural planting soils, and stormwater detention facilities integrated into the landscape and street medians. In addition, more than 200 trees were planted, contributing to the city's urban forest canopy goals.

Mercer Street includes custom-designed benches with concrete bases, Haskel stainless steel attachments and armrests, and Ipe wood seating surface from Compton Lumber. The wooden slats are oriented perpendicular to the sidewalk to act as skateboard deterrent without the need for skateblock additions. The green finger's LightWild light pavers feature a programmed sequence of alternating dark blue and white light that transition into each other. HBB Landscape Architecture worked with public artist Ellen Sollod to ensure the paver light pattern and color coordinate with the lighting of the tessellation sculpture and the corridor lighting design. At night, the paver lights and uplit elm trees set up a rhythm of lighting along the Mercer Street corridor.

Mercer Street
Now a high volume arterial with pedestrian connections and iconic gateway, Mercer Street was widened from a three lane, one-way arterial into a six-lane, two-way street with substantially planted edges, vegetated medians, and large-scale trees to create a green boulevard. The effect of the redesign nearly doubled the width of the sidewalk available to pedestrian use and reduces the perceived width of the street.

Sixteen large landscaped bulb-outs along the street edges, called "green fingers," support grand, boulevard-size trees and serve to delineate on-street parking stalls. The trees, among them maple, elm and cedar, are supported by large planting areas, increased soil volume, and structural soil channels. The trees calm traffic and provide a host of environmental benefits, such as improved air quality. Spaced periodically along the street, the green fingers integrate amenities such as custom-designed benches and LED paver lighting adjacent to the sidewalks to increase the vibrancy of the pedestrian realm.

Landscape plantings, including purple East Friesland salvia, help buffer the Seattle Streetcar at Lake Union Park Station on Valley Street. Textured and colored concrete designates pedestrian and bicycle crossing areas, with friendly reminders to watch for the streetcar before crossing. The integration of pedestrian walkways, bicycle paths, public transportation and vehicular roadways embodies "complete streets" principles.

Twenty-foot wide medians extend the length of Mercer St. and capture stormwater runoff from the adjacent pavement, directing the runoff into detention cells that alternate with mounded berms for the planting of trees. This functional role of the median reduces the project's impervious area by more than half an acre. The alternating pattern between tree berms and detention cells, with the contrasting textures of their plantings, and the steady spacing of the large trees, sets up a rhythm reinforcing a sense of flow as one moves along the corridor.

In addition, public art in the form of a large, tessellated column helps define the iconic character and industrial history of this streetscape. The sculpture, "Origami Tessellation," blends metal with internal light, providing an exclamation point at a vital gateway to the city of Seattle. Reflective in the day and luminous at night, it defines the focus of the surrounding neighborhood of technology companies, cancer research institutes, and biotechnology.

Looking at Valley Street from left to right, you see the Seattle Streetcar line with pedestrian and bicycle crossing paths; planted buffer; widened sidewalk as part of the south segment of Cheshiahud Lake Union Loop Trail; designated bike lane; and automotive lane. The sidewalks are buffered by feather grass plantings that wave and move with the passing of each streetcar. The plants' movement is reminiscent of the renovation's overall theme: the movement of people using a variety of transportation modes. HBB realigned the street so that eyes are naturally drawn to the brick and glass fa?_ade of the old Ford Assembly Plant, a century-old designated landmark building for the city of Seattle.

Valley Street
The design of Valley Street, a neighborhood street and promenade, provides a transition from the waterfront of Lake Union and the open spaces of Lake Union Park to the urban neighborhoods developing to the south, and links the Eastlake urban village with Queen Anne urban village. A large percentage of the space is given to the sidewalks, designated bicycle lanes, and transit routes. The street side parking is designed as an extension of the pedestrian sidewalks, both in surface pattern, finish grade, and material. Street trees are provided along the sidewalks and are integrated into the street side parking areas. The extensive plantings further soften the appearance of the street, extend the park into the street right-of-way, and blend the streetcar line into the transitional edges.

Pedestrian plazas provide seating at streetcar stops, entry points to Lake Union Park and the Museum of History and Industry, and trailheads for the Cheshiahud Lake Union loop trail. The project landscape architect championed preserving and strengthening the realignment of Valley Street so that the eastern terminus maintained an unobstructed view of the five-story, 1913 brick and glass façade of an old light truck assembly plant built for Ford Industries. Designed by Seattle architect John Graham Sr., the building is a designated landmark in the city of Seattle and a reminder of the neighborhood's industrial heritage.

Street-side parking on Valley Street, including custom wheel-stops and low-rise curbs, is integrated into the sidewalk paving and tree planting along the sides of the road. The widened sidewalk is part of the Cheshiahud Lake Union loop trail and connects to Westlake Avenue North.

Westlake Avenue North
Westlake Avenue North was reconfigured at its intersection with 9th Street to provide turn lanes, signals, and several pedestrian crossings. In cooperation with the Seattle Parks Department's landscape architect, the eastern edge of Westlake Ave. N. provides two pedestrian plaza spaces at entrances to Lake Union Park incorporating site furnishings, a distinctive grove of redspire pear trees, and sculptural artwork. A substantial portion of the streetscape was given over to rain gardens that detain and filter stormwater runoff flowing from over two miles of the street's length. The sinuous walkways define the edges of the rain gardens, while the axial pathways link the pedestrian plaza spaces to the park's interior. The result is a seamless transition from the urban streetscape to the park landscape.

The Mercer Corridor East project, encompassing the area of Mercer Street, Valley Street, and Westlake Avenue North, enhances pedestrian and bicycle mobility and safety; improves connections between neighborhoods and Lake Union Park; provides direct access to regional transportation corridors; supports east-west local and regional transit service; uses stormwater as a major feature of the landscape design; and fosters a walkable and healthy community with access for all. The design was developed through extensive public process.

Over two miles of newly planted rain gardens along Westlake Avenue North buffer the pedestrian and bike paths in Lake Union Park and the roadway. The paved paths are part of the Cheshiahud Lake Union loop trail.

This involved coordination between the design team, several neighborhood groups, Seattle Department of Transportation, Port of Seattle, King County Metro Transit, Seattle Design Commission, Seattle Public Utilities, Seattle Parks Department, Seattle Office of Arts & Cultural Affairs, and numerous stakeholder groups including the Gates Foundation, Vulcan, University of Washington, and Amazon Campus Headquarters. The completion of work on Mercer Corridor East marked a significant milestone in creating a livable, walkable, and workable neighborhood, while stimulating rapid redevelopment in a neglected district of the city. The project was awarded the American Public Works Association's 2015 Transportation National Project of the Year Award for the $25M to $75M division.

Materials List
LED Floor Tiles: LightWild
Column Lighting: B-K Lighting
Granite Stone, custom art bench: Arris Stoneworks
Metal Street Name In-Lays: Metallic Arts
Irrigation: RainBird
Wood, custom Green Finger Bench: Compton Lumber
Metal, custom Green Finger Bench: Haskel Constructors
Aluminum Landscape Edging: Alaskan Copper
Unit Paver Edging: Permaloc
Soils and Mulches: Pacific Topsoil
Compost: Cedar Grove Composting
Seed and Sod: Country Green Turf Farms

The entry plaza to Lake Union Park from Westlake Avenue North offers Hanover unit pavers, a grove of red spire pear trees, and site furnishings integrated with the park design. The paths through the park allow pedestrians and bicyclists easy access to the Museum of History and Industry located on the lakeshore.

Team List
Landscape Architect
HBB Landscape Architecture
Public Agency
Seattle Department of Transportation
Engineer
CH2M Hill Inc.
Urban Design
LMN Architects
Public Art
Sollod Studio
General Contractor
Gary Merlino Construction Company
Landscape Contractor
OutWest Landscape
Construction Manager
HNTB Corporation