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SR 509 Completion Project - Frequently Asked Questions

What will this project do?
This project will reduce congestion and travel time for people driving between Seattle and Tacoma by completing SR 509, which is an alternative to I-5 for this corridor.

The project will build a direct link between I-5, SR 509 and the south side of Sea-Tac International Airport between South 210st and SR 516.

The project will add lanes and make improvements along I-5 in south King County. The improvements will include a better interchange with SR 516 and new stormwater management features.

The project will also complete the SR 509 route and link it to I-5. New freeway lanes will be built to extend SR 509 three miles southeast from the point where it now ends near Sea-Tac Airport, connecting it with I-5 near S 210th St.

Why is this project needed?
The purpose of this project is to reduce travel time, congestion and accidents for people and freight on I-5 moving between Tacoma/south King County, Sea-Tac International Airport and Seattle’s industrial area and port.

Some specific needs are:

  • To provide an efficient route between the Kent Valley freight distribution hub and Seattle’s industrial area and waterfront.
  • To provide a direct connection to Sea-Tac International Airport for people and freight traveling on I-5 from the south.
  • To provide a better alternative route trucks that currently use the congested I-5 Southcenter Hill.
  • To support existing and new economic activity along the I-5 corridor.

How will the project benefit me?
Depending on where you live and travel, you could enjoy a number of benefits from this project:

  • Peak-hour travel time from Seattle to Tacoma will be shorter. Travel time, distances and costs will be reduced in many segments of the project corridor.
  • Freight shippers, businesses and people will spend less time traveling between south King County/Tacoma and Seattle.
  • South King County will be a better place for existing businesses and an attractive location for new ones—bringing jobs to the area.
  • Congestion will be reduced at the I-5/SR 516 interchange and along SR 516 and SR 167.
  • Traffic—and the air pollution that comes with it—will decrease on local city streets in Sea-Tac and Des Moines.
  • People traveling from the south on I-5 will have a closer connection to Sea-Tac Airport and the planned light-rail station.
  • Streams near the project will be enhanced, making them better habitat for salmon and trout.
  • The Des Moines Creek Park multi-use path will be extended.
  • SR 99 between S. 200th St. and S. 286th has been improved, as the I-5/SR 509 project worked in partnership with the city of SeaTac.

When will construction get started?
Construction funding allocated to this project starts in July 2019 and is distributed over a twelve year period.
After construction begins, how long will it take to build this project?
The construction funding allocated by the legislature is distributed over a twelve year period.

What is the plan for buying property?
Property must be purchased to clear right-of-way for this project. A total of 283 families (in single-family and multi-family residences), 25 to 29 businesses and two churches will be acquired.

The Washington Legislature has appropriated some funds to the project for purchasing property: $15 million from the 2003 5¢ gas tax, and $30 million from the 2005 9.5¢ gas tax, of which $24.5 million is being used to purchase property. The $15 million allocation has been used to acquire selected properties along the west side of I-5 between SR 516 and S. 212th Street, as well as some additional parcels. The $24.5 million allocated in 2005 will be used over the next three years to acquire several properties in the project area. To date, 91 properties have been acquired.

The state will pay fair market value for property, and will provide full relocation compensation and expenses.

The project team is committed to being sensitive to the unique needs of each family being relocated.

How can I find out when a particular piece of property will be purchased?
For information about your property, you may contact Andrew Lau, the project real estate manager, at or 206-768-5803.

Where can I get more information about WSDOT’s property-purchase policies and services?
General information about property purchases is available online. You can also contact Andrew Lau, the project real estate manager, at or 206-768-5803 with any other property related questions. 

Will noise levels increase as a result of this project?
Construction activities will result in short-term noise impacts. Contractors will take many steps to minimize noise, such as turning off idling equipment and confining work to daytime hours whenever possible.

In the long term, this project will cause noise levels to increase slightly along I-5.

Where will you build noise barriers?
WSDOT will install noise barriers at areas where residents would likely be affected by traffic noise and where construction of barriers is justified.

Noise walls will be constructed along most sections of I-5 to mitigate the increased noise along I-5. Location of proposed noise walls can be found in the Environmental Impact Statement (pdf 1 mb).

Won’t this project have an impact on traffic on the SR 99/First Ave. S bridge?
Peak-hour traffic on the First Ave. South bridge will increase only slightly as a result of the SR 509 project.

Traffic on the bridge will increase more during non-peak-hour travel times, as drivers will find that they can greatly reduce their travel times by taking the improved SR 509 route as an alternative to I-5.

WSDOT realizes that improvements will be needed to the intersection north of the First Avenue S bridge as a result of both the SR 509 project and existing limitations at this intersection. The next steps to address this need are getting this project listed on the Washington State Highway System Plan and starting the major I-5/SR 509 construction.  WSDOT will pursue both of these actions.

What effects will the project have on the local environment?
In the long term, this project will have a number of positive affects on the local environment. The project will:

  • Reduce air pollution on local streets by diverting traffic to the new expressway.
  • Contribute to the Marine View Drive bridge-replacement project, which removes a fish passage barrier and restores and enhances the stream bed and banks.
  • Contribute to the Des Moines Creek Basin capital improvement projects that will improve water quality and stream health.
  • Extend the Des Moines Creek Park bike path by one mile.
  • Improve facilities for storm-water management on I-5.

As the project is built, environmental impacts typical of a construction project will occur. WSDOT and its contractors will use best management practices and comply with all state and local regulations designed to minimize affects on the environment.

For detailed information see the final environmental impact statement for the project.

How will the existing section of SR 509 freeway function when the project is built?
During the peak afternoon travel time, motorists will experience some congestion on SR 509. However, the SR 509 route will remain less congested than I-5. Overall, the construction of the I-5/SR 509 project will result in reduced travel times between Seattle and Tacoma in the afternoon peak hours. 


Why has it taken so long to fund and complete this project?

While the SR 509 Completion Project has been planned for the region for decades, it only just recently received complete funding. Also, given that the Connecting Washington Funding Package releases portions of the funding every two years, design and construction activities stretch over the 16 year funding period.