I-90 over Mercer Slough
King County, Washington · rated Poor in the 2025 National Bridge Inventory
United States Bridge Conditions2025 National Bridge Inventory, FHWACompiled
- I-90 over Mercer Slough in King County, Washington, is rated in poor condition in the 2025 National Bridge Inventory (as of ).
- It was built in 1940 and reconstructed in 1970, making it 85 years old at the 2025 inventory.
- It carries about 74,410 vehicles a day (traffic count year 2023).
- Its listed owner is the state highway agency.
- A poor rating means at least one major component - deck, superstructure, or substructure - is rated 4 or lower on FHWA's 0-9 scale. It does not by itself mean the bridge is unsafe; bridges judged unsafe are closed or load-restricted by their owners.
View this bridge on a map → (47.57953, -122.19022)
Component ratings
| Component | Rating (2025) | FHWA scale meaning |
|---|---|---|
| Deck | 6 | Satisfactory condition |
| Superstructure | 4 | Poor condition |
| Substructure | 6 | Satisfactory condition |
Bridge details
| Structure | Continuous concrete multiple box beam |
| Length | about 2,812 ft |
| Location | 6.4 E Jct I-5 |
| NBI structure number | 0008610B0000000 |
Source: FHWA National Bridge Inventory, 2025 data as reported by the Washington department of transportation.
All bridges nearby: King County, WA bridge conditions · Washington bridges.
Background: how bridge ratings work · what "poor" and "structurally deficient" actually mean · how often bridges are inspected.