Fifteen Mile Creek Bridge No. 493C Replacement Project
Construction currently scheduled to begin in summer 2024
King County plans to begin construction on this project in summer 2024. The existing bridge will be closed for 10-12 weeks during the summer months of construction, and a temporary signed detour route will be in place.
Project area map
On this page
- Status update
- Why King County is replacing this bridge
- Getting through the construction zone
- Temporary detour route/alternate truck routes
- Project schedule
- Frequently asked questions
Broch Bender, Communications Manager
Follow us on Twitter
Find us on Facebook
Why King County is replacing this bridge
We are replacing the Fifteen Mile Creek Bridge because it is past its useful life. We have 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.
Improves safety by:
- Adding standard 8-foot shoulders on each side of the bridge for people walking, biking, and rolling.
- Increasing the height of the bridge railings by 14 inches, enhancing safety in the event of a collision.
- Raising the height of the bridge over the creek, providing more room for water to flow underneath during high-water events.
- The existing wooden bridge support piles are treated with creosote, which can be harmful to fish and the environment. This project removes the creosote support piles from the stream and replaces them with an out-of-water steel and concrete support system.
Maintains reliable routes for the community:
- The Fifteen Mile Creek Bridge replacement project is a long-term investment in route reliability, maintaining the connections between Issaquah, Maple Valley, and Renton. In 2009 and 2010, we also replaced two other bridges on the route connecting SR 900 and Issaquah-Hobart Road SE.
Getting through the construction zone
Work is scheduled to begin in summer 2024 to avoid the rainy season and minimize impacts on school bus service during the school year. We studied the option of building a temporary bridge during construction, but our studies showed that a temporary bridge would double construction time, be more expensive, and increase impacts on the environment, immediate neighbors, and the traveling public.
The existing bridge will be closed for 10-12 weeks during the summer months of construction, and a temporary signed detour route will be in place. The detour will add approximately ten (10) minutes to travel time.
Temporary detour route
The general traffic detour route will only be active during summer 2024 when the bridge is fully closed.
Alternate truck routes
Trucks are not allowed on roads in downtown Issaquah, including SE Newport Way, so they must use alternate routes such as State Route 169, State Route 18, Interstate 90, Cedar Grove Road SE, and SE 128th Street.
Frequently asked questions
6. What is King County doing to reduce the length of the construction closure and minimize impacts to the public?
The new bridge will raise the bridge deck higher from the stream bed than the existing bridge. Raising the bridge deck creates more space for the water and floating debris to move freely under the bridge, even during high-water events. The higher bridge deck also will reduce the risk of flooding onto the roadway and provide a larger opening for floodwaters to pass through, which will reduce flood levels along the stream.
The City of Issaquah is planning to reinstall their flood monitoring device on the new bridge. The flood sensor is part of a citywide network that helps to indicate where water levels are high and predict flooding during heavy rain or storms.