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I-5 - SR 509 Corridor Completion and Freight Improvement Project


November 2015

  • In Sept. 2013, WSDOT completed a study of the SR 167, SR 509 and I-5 Puget Sound Gateway Project which would relieve traffic congestion and improve freight mobility by completing the long-planned SR 509 and SR 167 corridor connections to I-5.
  • In 2010, WSDOT completed a SR 509 tolling study using a collaborative decision making process among the project partners.
  • With full project funding currently unavailable, WSDOT plans to complete this project in two or three phases.

Why is WSDOT extending SR 509?

Extending SR 509 will ease congestion on I-5, add a southern access to Sea-Tac International Airport and improve service between industrial districts by allowing general-purpose traffic and up to 9,000 trucks daily to bypass I-5, SR 99 and local streets.

What does the initial phase include?

  • A four-lane road between I-5/South 210th Street and 28th/24th Avenue South in Sea-Tac and a four-lane road between 24th/28th Avenue South and South 188th Street in Sea-Tac and Burien.
  • New interchanges at I-5/SR 509 and SR 509/28th Avenue South.
  • Additional collector-distributor lanes on I-5 from South 210th Street to SR 516.
  • A rebuilt I-5/SR 516 interchange, including a new connection to South S. 231st Way.
  • New southbound collector-distributor lanes from the new I-5/SR509 interchange to the S. 320th street interchange.
  • New southern access to Sea–Tac International Airport from SR 509 at 28th Avenue South.

The End Result

When finished, SR 509, the South Airport Access roadway, new I-5 lanes and improvements will become a key component of the Seattle and south King County transportation network. When considered in conjunction with the planned Alaskan Way Viaduct improvements, the project provides a critical north-south corridor alternative to I-5 through Seattle and South King County.

View a map of the project area.

Project Benefits

  • Congestion. Reduces congestion on I-5 in South King County by providing an alternate north-south route to I-5 while increasing I-5 capacity. 
  • Safety. Increasing capacity and improving traffic flow reduces the risk and frequency of accidents.
  • Freight. Provides a direct route for freight and general traffic movements to and from the Puget Sound marine ports and the industrial areas of Seattle and South King County.
  • Airport access. Provides for a new connection between I-5 and Sea-Tac Airport from the south.
  • Public value. By reducing travel times between Seattle and Tacoma by up to 12 minutes, the traveling public saves more than $100 million per year in travel time.   

What is the project timeline?

Once funded:

  • Obtain state and federal permits required for construction
  • Completed initial-phase design
  • Completed property acquisition
  • Continue utility-coordination efforts and relocation planning.

Financial Information

The 2008 cost estimate for the SR 509 initial phase is $1.2 billion to $1.3 billion. The latter estimate includes spent funds, the $35 million in new funding from the 2003 Nickel funding package and the $30 million in new funding received as part of the 2005 TPA funding package. Cost estimate range depends on when funding is available and inflation.

This project received funding through the following sources:

  • 2005 Gas Tax -$30 million
  • 2003 Gas Tax - $35 million
  • Other funds - $21 million
    (from pre-existing state, federal and other partnerships)
  • Funding available from all sources - $86 million
  • Unfunded - $1.2 billion

How can I get more information?

Amy Danberg, Public Information

Omar Jepperson, P.E.

The Washington State Department of Transportation is a public agency and is subject to the State of Washington’s Public Records Act (RCW 42.56). Therefore, public comments and questions may be made available to anyone requesting them for non-commercial purposes.

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