A critical initial segment of the new SR 509 Expressway, part of the extensive SR 509 Completion Project in King County, is expected to open to traffic this summer. This first mile of the new four-lane expressway is poised to significantly enhance regional mobility and provide a vital alternative to the frequently congested I-5 corridor in south King County.

This particular section, known as the "SR 509/I-5 to 24th Avenue South – New Expressway Project" or Stage 1b of the overall completion plan, involves the construction of the first mile of new SR 509 Expressway, along with new I-5 ramps, new interchanges, and a new bridge. While this segment is anticipated to open next year, some associated construction work will continue into 2026.

The SR 509 Completion Project is a key component of the broader Puget Sound Gateway Program, which also includes the SR 167 Completion Project in Pierce County. This comprehensive program aims to complete two of the Puget Sound region’s most critical freight corridors, improving access to I-5, the Ports of Tacoma and Seattle, and Seattle-Tacoma International Airport. When the SR 509 project is fully complete, it will establish an important north-south alternative to the congested I-5 corridor between Seattle and south King County.

Key benefits of the SR 509 Completion Project include:

• Improved Freight Connections: The project completes critical freight links between the Port of Seattle and key distribution, warehouse, and industrial centers in King County.

• Enhanced Regional Mobility: It is designed to reduce traffic congestion on local roads and highways by completing connections between Seattle and the Kent Valley, while also offering an alternate route to I-5 in south King County.

• Better Airport Access: The project provides a new connection between I-5 and Seattle-Tacoma International Airport (SEA Airport) from the south for both passengers and air cargo.

• Support for Regional Job and Economic Growth: The project supports regional job growth and economic expansion associated with the state's two largest seaports and SEA Airport.

• Intelligent Transportation: The SR 509 Expressway will feature electronic tolling to manage congestion, which is expected to improve safety and reliability for all users.

The entire Puget Sound Gateway Program, encompassing both the SR 509 and SR 167 projects, is supported by $2.83 billion in funding. The Washington State Legislature jointly funded these large projects in 2015 to maximize efficiencies in planning, environmental review, design, and construction.

While Stage 1b marks a significant upcoming opening, the full SR 509 Completion Project is a multi-stage endeavor with a total timeline stretching from 2015 to 2029. Stage 2, which began construction in late 2024, will build the remaining two miles of new SR 509 Expressway, reconfigure interchanges, and add an auxiliary lane on I-58. This final section, extending from 24th Avenue South to South 188th Street, is scheduled to open to traffic in 2028, with some construction work continuing into 2029. An earlier part of the project, Stage 1a, saw a new SR 99 bridge open to traffic in July 2022.

For drivers, it's important to note that the SR 509 Expressway will have electronic tolling. The Washington State Transportation Commission has issued its toll rate proposal for the SR 509 and SR 167 expressways, with a virtual public hearing on the proposed rates scheduled for June 17.