West Marginal Way SW Safety Corridor Project

Updated December 5, 2023

What's Happening Now?

Data shows more people biking & walking along West Marginal Way SW without impacting freight travel

Data collected between August and October of 2023 shows that the West Marginal Way SW Safety Corridor Project has led to several notable outcomes, including benefits for people biking, walking, rolling, and driving. We gathered this final set of data after installing a protected bike lane (PBL) along West Marginal Way SW (pictured below).

Three people riding their bikes along a protected bike lane along West Marginal Way

Data Highlights

  • Weekday bike volumes have increased by 144% from an average of 103 bikers before the project was installed to an average of 251 daily bikers. Weekend bike volumes have increased by 53%.
  • The amount of people walking/rolling (mobility assistance device or scooter) increased significantly, 96% on weekdays and 91% on weekends.
  • Overall traffic volumes are down significantly since the West Seattle High Bridge reopened and are on par with 2019 volumes at about 14,000 vehicles a day.
  • An analysis of southbound travel times indicates that since the repurposing of the southbound curbside travel lane into a PBL, there has been less than 1 second of additional delay.
  • A 7-day traveler interaction analysis between trail users and drivers moving in and out of the busiest intersections / industrial driveways showed very few instances of potential interactions. Less than 4% of all recorded interactions involved trail users. 

For information on these results, visit our blog, see our recent slide deck, or read the full report.

Next Steps

In the next phase of the project, we’ll install new ADA-accessible curb ramps and a permanent sidewalk along the Duwamish Longhouse crossing, scheduled to take place next year. With the sidewalk, we will also install new curb bulbs at the intersection of SW Alaska St and at a new driveway exit for the Duwamish Longhouse Cultural Center. Included with these improvements will be an art installation, of which a community process should begin in the new year to inform the artwork theme.

Later, we will upgrade existing ADA-accessible curb ramps and install a new crosswalk at the SW Front St and West Marginal Way SW intersection. This intersection has the highest volume of drivers crossing the Duwamish Trail and we want to ensure all travelers are more aware of each other at this trail crossing.

Project Overview

West Marginal Way SW connects communities and industrial businesses along the western bank of the Duwamish Waterway. It is a Major Truck Street between bridges and connects to Port of Seattle facilities. The street also provides direct access to the Duwamish Longhouse and Cultural Center, parks, and residences as well as shipping, manufacturing, and industrial areas. The Duwamish Trail, a multi-use recreational path for people walking and biking, runs alongside West Marginal Way SW for several miles.

The purpose of this project is to make West Marginal Way SW safer and more predictable for all travelers and maintain capacity and operations.

  • Safer = people driving at posted speed limits, designated space for people walking and biking, increased awareness at driveways and intersections
  • More predictable = defined space for all travelers
  • Close a crucial transportation gap = connect the Duwamish Trail and West Seattle Bridge Trail
  • Maintain capacity = maintain existing freight operations along corridor and to/from Port

Street Redesign

Diagram illustrating the planned street redesign for West Marginal Way Southwest. It includes a 6-foot walking path protected from the street a 5-foot strip with trees, next to that is an 8-foot bike lane protected from traffic by a 3-foot wide concrete barrier, they the road includes a 12-foot driving lane, an 11-foot center turning lane, two more driving lanes, on 10-feet wide and one 12-feet wide with an 8-foot wide planting strip next to it.

Moving freight and goods remains the priority for this Major Truck Street that serves port facilities, cargo yards, and major businesses. We carefully considered this distinction and the importance of maintaining freight reliability as we developed the new street design and implementation timeline.

The new design enhances safety for everyone by encouraging slower driving speeds, eliminating high-speed passing, improving visibility for people entering from side streets, and improving the predictability of all people using the West Marginal Way SW corridor. The project further separates vulnerable travelers (i.e., those who walk, roll, and bike) from faster-traveling traffic and trucks and connects the Duwamish Trail with The West Seattle Bridge Trail.

Protected Bike Lane 

As part of the street redesign, we installed a 0.4-mile two-way protected bike lane (PBL) and striping on the western side of the corridor between SW Marginal Pl and Duwamish Longhouse. We also updated all 17 Duwamish Trail driveway crossings on the east side of the corridor with signs and pavement markings.

Map showing the project area

Before the installation, we collected data on West Marginal Way SW four times and closed the southbound curbside travel lane for 0.4-miles of the 2.5-mile corridor (from SW Marginal Pl to the Duwamish Longhouse) for 2 weeks in Fall 2022 to better understand the traffic impacts to people driving cars and trucks. Find more information about these findings prior to installation on SDOT’s blog.

Highland Park Way SW and West Marginal Way SW Intersection Improvements

We improved the Highland Park Way SW and West Marginal Way SW intersection, making it more accessible, efficient, and safe. For this part of the project we:

  • Removed existing raised traffic islands and created a new northbound lane.
  • Shifted the southbound left-turn lanes farther west, providing enough space in the intersection to allow the westbound right turning vehicles (of all sizes) to go at the same time. This improvement will help to reduce wait times at the intersection. 
  • Updated the traffic signal equipment for people of all abilities and installed the infrastructure to allow us to make signal adjustments in real-time remotely from our Transportation Operations Center.
  • Installed new wheelchair-accessible curb ramps in compliance with the Americans with Disabilities Act (ADA).
  • Relocated the westbound bus stop.
  • Improved turning movements for people driving through the intersection to shorten wait times.

Improved Crossing at the Duwamish Longhouse

People driving fast on West Marginal Way SW can create safety challenges for people trying to access the Duwamish Longhouse. Thus, we installed a new, interim crossing signal that stops traffic for people walking and biking across West Marginal Way SW. A permanent crossing signal is being designed, along with a new permanent sidewalk with ADA compliant curb ramps.

Graphic showing the Duwamish Longhouse project on West Marginal Way

Traffic Mitigation

We made improvements to clarify roadway operations as well as encourage good driving habits.   

  • Added a traffic island at West Marginal Way SW in front of SW Front St.  
  • Added white, flexible posts to divide the right turn lane at West Marginal Way SW leading to the on-ramp to the 1st Ave S. Bridge.   

Traffic island and white flexible posts on West Marginal Way SW

(Left) West Marginal Way SW traffic island just north of SW Front St​ and (right) white, flexible posts to divide travel lanes

Driving Speeds 

We are committed to Vision Zero goals of reducing motorist speeding, crashing and fatalities. Throughout the project, we observed high speeds along West Marginal Way SW. To address this, we have implemented several measures aimed at reducing speeds along West Marginal Way SW. For example, we reduced the speed limit to 30 mph and installed Radar Feedback Signs to encourage slower speeds. After installation, speeds along the corridor declined by 12% from 43 mph to 37 mph on average.

Project History: Community & Data Driven

In March 2020 the West Seattle High Bridge, a vital lifeline connecting the West Seattle Peninsula to the rest of the city, was abruptly closed due to structural issues. In response to this unexpected disruption, we initiated the Reconnect West Seattle (RWS) program, in which community members identified and voted on priority projects. The West Marginal Way Corridor Safety Improvement Project was selected as a key priority based on both community feedback, and our vision for a safer, more accessible transportation corridor.  

The project's initial focus was on closing a gap in Seattle’s bike network by providing a protected bike lane (PBL) from SW Marginal Pl to just south of SW Idaho St and linking the Duwamish and West Seattle Trails. In response to concerns from local businesses, the project was expanded to analyze the entire corridor – from SW Marginal Pl to Highland Park Way SW. Careful about the impact to the area’s industrial businesses, we studied the corridor extensively, analyzing potential impacts to general purpose and freight traffic. We also delayed the opening of the PBL until after the West Seattle High Bridge reopened. In total, we have collected data along this roughly 2.5-mile corridor 13 different times since 2020.

We also consulted with stakeholders regarding the scope of data collection and analysis and used stakeholder input to inform project design and post-installation evaluation. Stakeholders included our Port of Seattle and maritime industrial community, bicycle and freight advisory boards, transportation safety advocates, local businesses, community groups and members, and the West Seattle Bridge Community Task Force. We also considered the Freight Master PlanBicycle Master Plan, our Vision Zero goals to inform our decisions. 

Ongoing Community Engagement  

We are committed to continue working with the community on enhancing safety along West Marginal Way SW and we will continue providing updates. Thank you to everyone who has helped us make this project better with your engagement.

Learn more, get involved, and tell us what you think:

Watch our project video, available in multiple languages:

Project Timeline

Early 2021

  • Engaged with area businesses, community partners, and neighbors.
  • Conducted outreach with businesses along West Marginal Way via online business survey.
  • Sent a mailer to West Seattle residents about how to give input. 
  • Held a virtual open house.

Spring 2021

  • Compiled and analyzed feedback

Summer 2021

  • Shared preferred design option with the community.
  • Announced decision to build protected bike lane after the high bridge reopens.

Fall 2021

  • Completed improvements at the West Marginal Way SW and Highland Park Way SW intersection.
  • Built a new sidewalk and finished the Duwamish Longhouse interim crossing signal.

August/September 2022

  • Data collection and safety assessment for all driveways and intersections along the east side of the corridor where they cross the Duwamish Trail

September 2022

  • Reopened the West Seattle Bridge
  • Data collection and analysis

October - November 2022

  • Conducted data collection of driver volumes/speed and walking/biking counts on West Marginal Way SW from October 23 to November 6.

December 2022

  • Duwamish Trail vegetation overgrowth clearing

January 2023

  • Temporary Protected Bicycle Lane (PBL) installed for the Low Bridge detour route
  • Data collected

February 2023

  • Conducted public engagement for the project

April-May 2023

  • Installation of a two-way protected bike lane and striping on the western side of the corridor between SW Marginal Pl and Duwamish Longhouse

Summer 2023

  • Monitor project area and evaluate performance

Fall/Winter 2023

  • Report back on data collection findings


  • Update the temporary sidewalk to a permanent sidewalk with curb bulbs and ADA-accessible curb ramps. Included with these improvements will be an art installation, of which a community process should begin in the new year to inform the artwork theme.


  • Upgrade existing ADA-accessible curb ramps and install a new crosswalk at the SW Front St and West Marginal Way SW intersection

Project Materials

Update to community (May 2022)

Bike lane decision blog announcement (7/14/21)

Watch the open house on YouTube (2/18/21)

Open House PowerPoint Presentation (2/18/21)

Open House Summary (2/18/21)

West Seattle Transportation Coalition Meeting Presentation (1/28/21)

Project mailer (English - 1/21) 

Translated project information

West Marginal Way Business Survey (2/4/2021)

Past Outreach Activities


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.