We may have rounded the corner to summer and can finally (hopefully?) put away our raincoats and puffy jackets until fall. But there are bigger changes than the seasons coming to the South 188th Street/State Route 509 interchange as part of the SR 509 Completion Project in south King County.
Upcoming changes to SR 509 Interchange
Scheduled to begin Monday, July 7, crews will change the SR 509 ramps to and from eastbound South 188th Street/Des Moines Memorial Drive South and install a new traffic signal. Drivers will use the new traffic signal to:
- Enter northbound SR 509 from both directions of South 188th Street/Des Moines Memorial Drive South.
- Exit southbound SR 509 to eastbound South 188th Street/Des Moines Memorial Drive South.
People driving from southbound SR 509 to westbound South 188th Street/Des Moines Memorial Drive South will continue to use the existing off-ramp. This change will be in place for the next two years.
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| Starting July 7, crews will change the SR 509 ramps to and from eastbound South 188th Street/Des Moines Memorial Drive South. |
Changes necessary for safety and efficiency
These changes to the SR 509 and South 188th Street/Des Moines Memorial Drive interchange are a key step toward building the new SR 509 Expressway between South 188th Street and 24th Avenue South. This work is important to make the construction area safer and more efficient for our crews. Once this section of the expressway is complete, we’ll begin major upgrades to the interchange, including new roundabouts and a multi-use path that will link directly to King County’s Lake to Sound Trail.
If you live near, work near, or travel through this area, expect noise and dust typically associated with road construction. Trying to reach local businesses? Don’t worry, we’ll make sure you can still get there.
Short-term inconvenience for long-term benefits
We know traffic changes are a hassle, but we promise the short-term inconvenience will pay off in big ways down the road (pun intended). Once complete, the improvements to the South 188th Street interchange and the new SR 509 Expressway will reduce congestion, streamline traffic flow, and boost safety in the area, making it easier for everyone to get around.
This is all part of the larger SR 509 Completion Project, which:
- builds a new four-lane expressway between Interstate 5 and SR 509's current end near Sea-Tac Airport
- builds new I-5 ramps
- improves I-5 interchanges in south King County
- constructs new bridges
When the entire SR 509 Completion Project is done, it will create an important north-south alternative to the congested I-5 corridor between Seattle and south King County. The future is bright!
New interchange improves access
When all is said and done, the new SR 509/South 188th Street interchange will let people travel in all directions, improving access for everyone—whether you’re heading north, south, east or west. The design will include roundabouts, which are safer and more efficient than traditional intersections. Plus, for those heading south on SR 509, the South 188th Street interchange will be the last exit before the expressway’s tolled section.
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| When the SR 509/South 188th interchange work is completed, people will be able to travel in all directions with improved access whether you drive, walk, ride or roll through the area. |
Why roundabouts?
Because they work—and they work well. Our interchanges are designed to be safer for all road users, including drivers, pedestrians, and cyclists. Roundabouts help slow down traffic, provide safer crosswalks, and reduce the chances of dangerous collisions. They also keep traffic moving smoothly and efficiently, even as volumes increase.
For this project, we ran traffic studies and simulations to determine the safest and most effective design. The roundabouts came out on top, outshining other options, especially when we factored in large vehicles like trucks and buses, as well as non-motorized users.
Roundabouts are already working wonders at over 100 locations across Washington State. We’ll continue share information roundabout safety, so everyone feels confident navigating them once construction is complete.
Have questions or want more information?
We know that change brings questions, and we’re here for them! Send us an email at sr509construction@wsdot.wa.gov or give our 24/7 construction hotline a call at 206-225-0674.
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Thanks for your patience as we work to make this important project a reality. We’re excited for the positive changes that lie ahead!
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