Beginning next week, King County Road Services will begin work to replace Fifteen Mile Creek Bridge #493C near Issaquah.

The job will require a 10-week closure of the bridge beginning in mid-June along SE May Valley Road near Issaquah-Hobart Road SE that will require a detour for both general and truck traffic.

Below are the details:

Project overview

King County is replacing the Fifteen Mile Creek Bridge because it is past its useful life. It has repaired this bridge many times over the years, and now it is time to replace it.

The new bridge will be designed to meet current national bridge standards. These standards include weight capacity, lane and shoulder widths, and environmental protection.

Why now?

The existing bridge was constructed in 1932 – that’s 94 years ago. The timber bridge has been treated with creosote, which typically lasts about 50 years.

Due to its age and condition, it can no longer be safely repaired or maintained, and a full replacement is required.

King County Road Services Division has coordinated with the Issaquah School District on this project for many years. While our goal is always to avoid any overlap with the school schedule, the district has consistently indicated that if overlap is unavoidable, they prefer it to occur at the end of the current school year rather than at the beginning of the next. They reaffirmed this preference during our February meeting and in follow‑up communications.

Road restrictions, then closure

The full removal and replacement of the bridge will take up to 12 weeks. The road will be reduced to a single lane, with flaggers directing two‑way traffic through the work zone at the bridge location from 7 a.m. and 4 p.m., June 2-12.

The full closure will begin as early as 7 a.m., June 15, and traffic on SE May Valley Road will be detoured. There are separate detour routes for general and heavier truck traffic (see maps below)


General detour route: All traffic, including emergency vehicles and school buses, will follow the signed detour route during the closure. Trucks over 15,000 Gross Vehicle Weight (GVWR) will need to use an alternate detour route.
General detour route: All traffic, including emergency vehicles and school buses, will follow the signed detour route during the closure. Trucks over 15,000 Gross Vehicle Weight (GVWR) will need to use an alternate detour
route.
A map of where alternative truck traffic will be detoured during the closure of the Fifteen Mile Creek Bridge.
Alternate truck routes Large freight trucks are generally restricted from using local City of Issaquah roads (e.g., Front Street S.) unless making local deliveries, with through-traffic directed to SR 18 or I-90. Heavy, oversized loads (exceeding 8’6″ wide, 14′ high, or 80,000 lbs) require permits, and significant restrictions apply during peak commuter hours.

Our goal is to reopen the road the evening of Aug. 23, prior to the start of the 2026-2027 academic year. This schedule is extremely tight and must comply with environmental regulations and permit requirements that dictate when work can occur. Dates may shift due to weather or crew and equipment availability, but this timeline provides the best opportunity to complete the project before the next school year begins. Any intentional delay would risk extending the closure.

While there is never an ideal time for roads to be closed, we look forward to delivering a safer and more reliable corridor for everyone who uses it.

Are impacted schools aware of the project?

In addition to keeping the district informed of our schedule, we have contacted impacted schools directly to ensure they have the information and resources they need. While the district is responsible for distributing information through its communication channels, we are partnering closely with them to help ensure staff, students, and families are aware of the closure and can plan ahead. To inform the broader community, we installed construction signs in February that included informational flyers, mailed a postcard to approximately 14,500 properties, and hosted an online community meeting. Additional signage will be installed soon.

Will there be any impact to emergency response times because of the project?

We have been working closely with Eastside Fire & Rescue, Bellevue Fire, Puget Sound Regional Fire Authority, and the Renton Regional Fire Authority. The Eastside Fire & Rescue deputy chief of operations has asked us to reassure community members near the bridge and in the Mirrormont community that they do not expect any change to emergency response times and that their coverage remains strong. Even with the bridge closed, we have been told three other stations in the Issaquah area are positioned to reach these neighborhoods quickly. 

They also shared that before Station 78 opened in 2016, Station 71 was already the primary responding engine for Mirrormont, and that capability remains in place. For major incidents, the regional system will continue to automatically dispatch the closest available units, including support from their partners in Puget Sound Fire. That coordinated response model will not change during the closure.  In addition, Station 76 within Mirrormont will host Department of Natural Resources wildland crews this summer, providing extra seasonal coverage and helping reduce concerns about wildfire response.

For more information

Fifteen Mile Creek 2026 Project FlyerDownload