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Fifteen Mile Creek Bridge No. 493C Replacement Project
Construction began on June 15, 2026
Fifteen Mile Creek Bridge No. 493C connects Issaquah, Renton, and Maple Valley. The bridge is located between Tiger Mountain State Forest and Squak Mountain State Park Natural Area.
Status update
June 15, 2026
On Monday, June 15, 2026, at 7 am, crews closed SE May Valley Road for replacement of the Fifteen Mile Creek Bridge #493C. SE May Valley Road near Issaquah Hobart Road SE is closed in both directions and a temporary signed detour route is in place. The road is closed to all vehicles, including emergency responders, pedestrians, and bicycles.
Please note that 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. Please see below for alternative routes for trucks.
The closure is expected to remain in place through midnight on August 23, 2026. This work may be delayed due to crew or equipment availability or weather conditions.
View the project flyer (4MB)
View the slides from the Community Meeting on May 12, 2026 (6MB)
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Project area map
Map showing the Fifteen Mile Creek Bridge Replacement Project location on SE May Valley Road, just west of the intersection with Issaquah-Hobart Road SE. Larger view of the map (1MB)
Why King County is replacing this bridge
Larger view of the cross-section graphic (361KB)
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 is 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.
Environmental benefits:
- 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
Construction began on June 15, 2026, 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 bridge is closed for 10-12 weeks during construction, and a temporary signed detour route is in place.
Temporary detour route
The general traffic detour route is in place now that SE May Valley Road is closed for construction.
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.
Project schedule
The road closure to replace the Fifteen Mile Creek Bridge #493C began on June 15, 2026. The work will take approximately 10 weeks to complete.
- Advertised project to contractors: 2025
- Construction started: June 15, 2026
- Major work complete: late-August 2026
Budget and funding
The estimated total project budget is $6 million, which includes engineering studies and design, environmental documentation and permitting, right-of-way acquisition, construction, inspection, and project closeout. The project is funded with a federal bridge grant.
Frequently asked questions
The new bridge will be 38 feet wide, with two 11-foot-wide travel lanes and two 8-foot-wide shoulders. The span (length) of the new bridge will be 65-feet-long. The longer span will allow the new concrete bridge footings to be located outside of the stream channel. The existing bridge is 27-feet-wide and 38-feet-long.
The new bridge will be built in the same location as the existing bridge. This project will not change anything about the location of the road or the bridge.
We are closing the bridge for construction during the summer for three main reasons.
- There is more daylight, temperatures are warmer, and there is less rain, which allows our contractor to get more work done during the day.
- This time of year, is also known as the fish window, which is the period when it is safe to perform in-water construction work without disrupting salmon migration.
- Closing the road during the summer allows us to keep the road open during the school year, which helps buses and people driving children to school avoid detour delays.
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 a 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.
King County Road Services Division has 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 Mirrormont that they do not expect any change to emergency response times and that their coverage remains strong. Even with the bridge closed, King County has been told three other stations in the Issaquah area are positioned to reach nearby neighborhoods quickly.
Eastside Fire & Rescue also shared that, before Station 78 opened in 2016, Station 71 was the primary responding engine for the Mirrormont community. Throughout the SE May Valley Road closure, TriMed Ambulance will post an additional unit at Station 71 each day from 6 am to 8 pm. During this period, TriMed will handle transports for the area, which will help keep more Eastside Fire & Rescue units available to serve the community.
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 the Washington State Department of Natural Resources wildland crews this summer, providing extra seasonal coverage and helping reduce concerns about wildfire response.
Photos
February 2026: Crews added blue netting under the bridge to prevent birds from nesting. The current structure will be demolished during construction and we don’t want any bird nests to be harmed.
The existing bridge does not have shoulders. The new bridge adds two 8-foot shoulders to the roadway.
The existing bridge has a pedestrian sidewalk on one side of the bridge. The new bridge will have pedestrian pathways on both sides of the bridge.
The wooden guardrail posts are aging and worn out. The project will replace the old wooden posts with a new, all-steel guardrail system.
King County Road Services installed steel supports under the existing bridge to help strengthen the old, worn-out timber cross beams.
Some of the timber support beams on the existing bridge are worn out. The new bridge replaces the timber beams with concrete bridge foundations.
From this view of the bridge looking south, see the creosote-treated timber piles which are touching the water.