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SE Green Valley Road at 218th Ave SE Roundabout Project
Construction of the intersection improvement project is anticipated for 2030.
Drivers often do not stop at the all-way stop signs at the intersection of SE Green Valley Road and 218th Ave SE/212th Way SE. King County plans to replace this existing four-way stop with a new roundabout to keep traffic flowing safely.
Status update
Fall 2025
Construction to build a new roundabout at the intersection of SE Green Valley Road and 218th Ave SE is anticipated for spring/summer 2030. These dates can shift due to permit requirements, funding, or crew availability.
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Project Overview
King County plans to build a single-lane roundabout at SE Green Valley Road and 218th Avenue SE, southwest of Black Diamond, to improve safety and traffic flow. A four-way stop was installed in 2021, but many drivers still miss the signs and drive through, causing frequent crashes. A roundabout will help slow traffic, and reduce collisions, making the road more welcoming and safer for all kinds of vehicles.
The project will also add wider shoulders, which gives more space for those who roll, use bicycles or wheelchairs, and pedestrians. New raised curbs and crosswalks will also make it easier for everyone to cross the street safely.
Drivers in the area are already familiar with roundabouts. Black Diamond has several and another is planned for SE Auburn Black Diamond Road and 218th Avenue SE.
Construction is anticipated to begin in 2030.
Why is a roundabout needed in this location?
The intersection at SE Green Valley Road and 218th Avenue SE/212th Way SE has seen an increase in traffic in recent years. From 2015 to 2019, traffic heading north and south increased by about 24%, with many of those vehicles being large farm trucks that take up lots of space and need more time to stop. In 2021, King County added stop signs on all sides to try to make things safer, but some drivers still miss the signs and drive through without stopping.
Building a roundabout here will slow down traffic, help drivers pay more attention, and reduce the types of crashes that often happen at intersections. Roundabouts keep cars moving at slower speeds and eliminate dangerous right-angle and left-turn accidents. Since there are already other roundabouts in Black Diamond, most drivers are familiar with how to use them. Plus, a second roundabout is planned nearby at SE Auburn Black Diamond Road and 218th Avenue SE, helping to further improve safety in this growing area.
How to use the roundabout
All vehicles may enter the intersection from all four entry and exit points of the roundabout. Drivers enter the roundabout by yielding to traffic that is already in the circle and drive in a counterclockwise direction until they reach their desired exit point.
This graphic shows the layout of a planned roundabout, featuring a central island and clearly marked lanes that guide vehicles through the intersection safely and efficiently. With four entry and exit points, the design helps slow traffic, reduce collisions, and accommodate large trucks common in the area.
Extra room for large trucks
The central island includes a truck apron, a raised section of red concrete that acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn.
Pedestrians and non-motorized vehicles
Signed crosswalks and separated shoulders allow people to safely cross from one side to another. Openings in the raised curb will make it easier for bikes, strollers, and wheelchairs to use the crosswalks.
Frequently asked questions
King County looked at three options before selecting the roundabout design:
- Make no change
- Build a compact roundabout
- Build a single-lane roundabout (selected design)
