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Project Info

SR 509 Extension Phasing Plan

The SR 509 Extension and Interstate 5 Improvement Project has been developed through many years of cooperative efforts between WSDOT, the Port of Seattle, King County, and the cities of Sea-Tac, Des Moines and Kent. The project proposes critical freight and mobility improvements for SR 509 including completion of the planned SR 509 corridor extension to I-5, improvements on I-5, and more direct access to Sea-Tac International Airport and the Kent Valley.  Thumbnail map of SR 509 preferred phasing option
Map of SR 509 Extension preferred phasing plan

WSDOT received funding from the Transportation Research Board's Second Strategic Highway Research Program (SHRP 2) to test the Transportation for Communities—Advancing Projects through Partnerships (TCAPP) tool. TCAPP provides a systematic approach for reaching collaborative decisions about adding highway capacity that improves transportation while enhancing the environment, the economy, and the community.

Building on the SR 509 Extension Toll Feasibility Study completed in 2010, WSDOT and its partners will use the TCAPP decision making tool developed by the SHRP 2 program to move the project forward.

Why is WSDOT planning the phasing of the SR 509 extension?

Although the project is expected to provide benefits for passenger and freight movements, project funding has not materialized. Many attempts in the past, including the Regional Transportation Investment District, were made to fund the project, but were not successful. One of the key findings from the SR 509 Extension Toll Feasibility Study is that tolling would not only generate revenue, but would also help manage travel demand creating an opportunity to phase construction and reduce project costs. Another key finding from the toll feasibility study is that under the most favorable financial conditions, toll revenue could fund a  portion of the construction cost of a scaled down, optimally phased project.

The End Result

Using TCAPP collaboration techniques, WSDOT worked with project partners to develop a preferred phasing option for consideration by the state legislature. Included in the preferred phasing option is:

  • One lane in each direction on SR 509 between S 188th Street and S 24th/28th avenues
  • Two lanes in each direction on SR 509 between S 24th /28th avenues and I-5
  • Full diamond interchange at SR 509 and S 188th Street
  • Half diamond interchange at SR 509 and S 24th Avenue. This option provides direct access to the airport via S 24th and 28th avenues and an improvement over the existing Air Cargo Road from S 188th Street to the airport.
  • Conversion of the existing I-5 HOV lane to express toll lane lanes and the use of inside shoulders as a second express toll lane during peak periods from the SR 509 connection to Federal Way.
  • New interchange at 509 and I-5 that provides direct access to I-5 general purpose lanes and express toll lanes.
  • A new connection between I-5 and Kent Valley via S 228 Street
  • Reconstruction of the I-5 and SR 516 Interchange with collector-distributor connections between SR 516 and S 228th at I-5.

The preferred phasing option does not toll the existing SR 509 (north of S 188th Street). The project partners also acknowledge that although converting I-5 HOV lanes to express toll lanes is consistent with the state and Puget Sound Regional Council’s regional transportation plan, it is a major policy decision that requires further analysis and legislative action.

Contact Us
Shuming Yan
WSDOT Urban Planning Office
206-464-1276
urbanplanning@wsdot.wa.gov