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Boise X Connection Bridge #3055A Replacement Project

Project expected to start in summer 2025

Boise X Connection Bridge.

The Boise X Connection Bridge over Boise Creek was built in 1956. It got its name from an old, elevated Northern Pacific Railroad trestle that used to be adjacent to it.

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Project overview

This project replaces the Boise X Connection Bridge located two miles south of Enumclaw, WA on SE Mud Mountain Road. This two-lane bridge was built in 1956 and has outlived its useful life. The bridge is weight restricted and cannot safely support the weight of heavy fire engines and large commercial trucks.

Road Services will remove the existing bridge and replace it with a 67-foot-long, 34-foot-wide, two-lane reinforced concrete bridge in the same location. The new bridge will not have any weight restrictions. The project also improves sight distance for drivers by lessening the severity of a noticeable curve in the road approaching both directions of the bridge.

The road will be closed to all traffic for five to six months during construction. The entire project is expected to start in summer 2025 and end in late fall 2025.

Graphic of the new bridge cross-section.

Project map

Map of the project site.

Larger view (87KB)

Why is King County replacing the bridge?

Replacing the bridge provides a safer, reliable crossing for the community.

The existing Boise X Connection Bridge was designed for much smaller vehicles compared with what we see on the road today. The bridge is nearly 70 years old. It has outlived its useful life and is showing its age in the following ways:

  • Original steel deck and steel girders are rusting.
  • Creosote treated timber foundations are rotting.
  • Lead-based paint is peeling off.
  • The bridge is weight restricted—large trucks and other heavy vehicles cannot use this bridge.
  • The bridge is too narrow for the size of cars and trucks that use the road.

The replacement bridge improves driver safety and the surrounding environment. This project also includes a public art installation, funded by King County's 1 Percent for Art program.

Safety improvements

  • The new bridge will be built to modern safety design standards.
  • It will be open to all vehicles, regardless of size or weight.
  • The curves in the roadway approaching both ends of the bridge will be slightly straighter to make it easier for drivers to see further ahead (sight distance).
  • The new bridge will have standard 11-foot-wide travel lanes, 6-foot-wide shoulders, and modern bridge railings and guardrails on both sides.

Environmental improvements

  • Removes decaying creosote timber support piles that leach toxic chemicals into the soil and water.
  • Eliminates the source of lead paint currently peeling off the bridge and falling into the stream.
  • Relocates the bridge supports further from the stream, giving the stream extra room to move.
  • Installs large woody material and new streambed gravels to improve fish habitat.
  • Removes invasive plants and restores the area with native shrubs and trees.

What to expect during construction

Construction activities will include demolition and removal of existing bridge, concrete pouring, and crane lifts. After the new bridge is opened to traffic, crews will continue low impact restoration work including plantings and site clean-up.


Construction to replace the Boise X Connection Bridge is anticipated to begin in summer 2025. The work will take approximately six to eight months.

  • Advertise project to contractors: Winter 2024
  • Construction starts: Summer 2025
  • Major work complete: Fall 2025

Detour map

The Boise X Connection Bridge on SE Mud Mountain Road, east of State Route 410 (Enumclaw-Buckley Road) will be closed for up to six months during construction. All traffic will follow a posted detour when the road is closed.

Larger view (150KB)


The preliminary cost estimate for the project is $4.7 million. Funding sources include grants from the Federal Highway Administration (FHWA) and local King County matching funds.


Steel floor beams.

The steel floor beams of the existing Boise X Connection Bridge are rusted and continue to age and deteriorate. King County will remove the entire bridge and replace it with a new concrete bridge. (Photo: July 2019)

The county has repaired and reinforced the decaying creosote timber bridge supports many times over the years. The creosote supports leach toxic chemicals into the environment. In addition, the large, jagged rocks and pieces of concrete shown above are not healthy habitat for fish and wildlife. (Photo: July 2019)

This is a photo of SE Mud Mountain Road approaching the bridge from the east. The curve in the road limits the ability to see traffic ahead, especially at night. The county will lesson the severity of this curve to improve sight distance for drivers. (Photo: March 2021)

The existing Boise X Connection Bridge is restricted for heavy trucks and large fire engines. The new bridge will not have any weight limits. (Photo: July 2019)

Photo of Boise X Connection Bridge (foreground) when it first opened in 1956. The Northern Pacific Railroad trestle in the background was torn down in 1982.


Frequently asked questions

Yes. The bridge remains safe and open for passenger cars and trucks. King County will continue to inspect and maintain the existing bridge regularly to ensure it is safe to use.
The county restricts specific heavy trucks, very large fire engines and specialized hauling vehicles. Weight limits are posted near the bridge. Weight limit signs will remain in place and heavy vehicles will continue to be restricted until the new bridge is complete.
The project will replace the bridge in the same location on SE Mud Mountain Road, east of State Route 410 (Enumclaw-Buckley Road).
No. The new bridge will be built to current standards and will not have vehicle weight or size restrictions.
There will not be any access for drivers, pedestrians, or emergency vehicles when the new bridge is under construction. The project will post a signed detour directing traffic around the closure.
The preliminary cost estimate for the project is $4.7 million. Funding sources include grants from the Federal Highway Administration (FHWA) and a local King County funding match.
The project will remove, transport, and dispose of the existing and creosote-treated timber piles at an approved disposal facility. Most of the rusted steel and concrete will be recycled.
The project will not restrict access to any public trail or park. Access to Lower Boise Creek Trails and Foothills Trail may be impacted or temporarily restricted during construction.