WSDOT Projects

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Artist's rendering of the landscape and lighting design for the SR 518/SR 509 interchange improvements.


View enlarged image. Potential SR 509/ SR 518 interchange alternative developed for the Route Development Plan (pdf 18 mb).

Project Facts

• 80% of traffic to SeaTac International Airport travels on SR 518.
• Due to airport traffic, SR 518 carries more traffic than the SR 520 Bridge.
• Airport passenger growth is projected to increase from 29 million annual passengers today to 44 million in 2020.

Why is WSDOT pursuing this project?
Forecasts call for strong air travel growth over the coming years, which will dramatically intensify traffic on SR 518.  If nothing is done over the next five years, drivers leaving the airport will experience more than 30-minute delays with lines of cars backing up nearly to the airport garage.

Two primary interchange projects make up the SR 518 Corridor Improvement Projects.  These projects are in the planning phase:

SR 509/SR 518 Interchange Improvement Projects -This project will evaluate an SR 509 southbound to eastbound freeway-to-freeway connection to address existing access and safety problems and provide for anticipated growth.

SR 518 Sea-Tac Airport to I-5/I-405 Interchange Project - This project will improve existing mobility and safety as well as accomodate projected airport traffic by adding a third eastbound lane between the North Airport Expressway and the I-5/I-405 interchange.

Our Partners
The Port of Seattle, the City of Burien and WSDOT have formed a partnership to lead this project.  WSDOT is working closely with the Port and the City of Burien to coordinate construction of the improvements.

The End Result

• Builds a southbound to eastbound freeway-to-freeway ramp.  The ramp could cross under SR 518, then over SR 509, or the ramp could cross over both SR 518 and SR 509.
• Replaces the existing diamond/loop interchange with a three-quarter single point urban interchange.
• Widens SR 518 by adding a third eastbound lane between Sea-Tac International Airport and the I-5/I-405 interchange.
• Provides for further regional improvements, including Sound Transit’s LINK light rail line extension to the airport and the Airport traffic and parking enhancements slated for completion in 2006, which are both dependent on the SR 518 improvements.

Project Benefits

• Congestion Relief. The projects reduce airport congestion and delays.  Adding a third lane will result in better traffic flow during airport and freeway rush hours and will yield travel time savings at the airport ramps that currently experience backups.  The new flyover ramp will make it easier to get to the airport and provide more direct access to the city of Burien.

• Safety. There are a lot of accidents at the intersection of SR 509 and SR 518.  The new ramp will reduce the number of accidents and improve traffic movement.  Adding a third eastbound lane will simplify the entrance to I-5 and I-405, therefore increasing safety.

What is the project timeline?

• Fall / Winter 2006. The project office is continuing the design for improvements to the SR 509/518 interchange and eastbound SR 518 from Sea-Tac Airport to the I-5/I-405 interchange. The Environmental Assessment for the improvements on SR 518 from Sea-Tac Airport to the
  I-5/I-405 interchange has been circulated for public and agency review and is anticipated to be finalized in late 2006.
• Spring 2007. Improvements to the SR 509/518 interchange and eastbound SR 518 from Sea-Tac Airport to the I-5/I-405 interchange will be advertised for construction.

Public Involvement
Your involvement throughout this project is important.  It ensures that problems and concerns regarding alternatives and foreseeable impacts are addressed.

Send your thoughts, comments, suggestions or questions directly to our project office by filling out the Contact Us form.

Environmental Protection
The Environmental Assessment (EA) will identify and evaluate potential effects of the project.  Of particular interest are environmental issues such as transportation, land use, wetlands, geology and soils.

Please visit the WSDOT Environmental Services Web site for more information.

Increasing safety is one of our priorities
Please see "Project Benefits" section above for more information on the project's safety improvements. 

Will this project impact tribal resources?
At WSDOT we seek to address the concerns of the tribal nations using the process outlined in Section 106 of The National Historic Preservation Act and the WSDOT Tribal Consultation Policy adopted in 2003 by the Transportation Commission as part of the WSDOT Centennial Accord Plan.

Financial Information
The SR 518 corridor improvement projects have received $5 million in Federal and Port of Seattle funding to develop preliminary designs and assess environmental conditions for the two proposed projects.

See the Project Status box above for information about the recent funding that we have received through the Transportation Partnership package.

How can I get more information?
Contact:

Paul E. Johnson, P.E.
Project Engineer
WSDOT Urban Corridors Office
Phone: 206-768-5680

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