I-90 over Mercer Slough
FairKing County, Washington · 6.4 E JCT I-5 · Structure no. 0008610C0000000
- Built
- 1970
- 55 years old, rebuilt 1993
- Condition
- Fair
- inspected February 2023
- Type
- Slab
- Concrete Continuous
- Length
- 1,723 ft
- 42 main spans
- Daily traffic
- 31,000
- 2013 count
- Owner
- State
- State Highway Agency
About this bridge
I-90 over Mercer Slough is a concrete continuous slab carrying I-90 over Mercer Slough in King County, Washington. It was built in 1970 and reconstructed in 1993, so it is 55 years old, older than the Washington average of 49.8 years.
The most recent inspection reported to the Federal Highway Administration rates it in fair condition with a lowest component rating of 5 out of 9. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.
It carries about 31,000 vehicles a day, roughly 5% of them trucks, projected to reach 56,912 by 2034. The structure is 1,723 ft (525.2 m) long and 52 ft wide. If it were closed, the detour would be 1.2 mi (2 km).
Location
Open in Google MapsCondition and inspection
The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.
| Component | Rating | Meaning |
|---|---|---|
| Deck | 6 Satisfactory | Structural elements show some minor deterioration. |
| Superstructure | 5 Fair | All primary structural elements are sound but may have minor section loss, cracking, spalling or scour. |
| Substructure | 6 Satisfactory | Structural elements show some minor deterioration. |
| Channel and channel protection | 8 Very Good | No problems noted. |
Structure
Traffic and service
Ownership and planned work
Frequently asked questions
When was I-90 over Mercer Slough built?
It was built in 1970, making it 55 years old in 2025. It was reconstructed in 1993.
What condition is I-90 over Mercer Slough in?
The National Bridge Inventory rates it in fair condition. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound. The last reported inspection was in February 2023.
How much traffic does I-90 over Mercer Slough carry?
About 31,000 vehicles per day (2013 count), of which roughly 5% are trucks.
How long is I-90 over Mercer Slough?
The structure is 1,723 ft (525.2 m) long with a longest span of 42 ft (12.8 m) and a deck 52 ft wide.
Who owns I-90 over Mercer Slough?
It is owned by the State Highway Agency.