I-90 over Mercer Slough
PoorPosted for loadKing County, Washington · 6.4 E JCT I-5 · Structure no. 0008610B0000000
- Built
- 1940
- 85 years old, rebuilt 1970
- Condition
- Poor
- inspected February 2023
- Type
- Box Beam or Girders - Multiple
- Concrete Continuous
- Length
- 2,812 ft
- 7 main spans
- Daily traffic
- 74,410
- 2023 count
- Owner
- State
- State Highway Agency
About this bridge
I-90 over Mercer Slough is a concrete continuous box beam or girders - multiple carrying I-90 over Mercer Slough in King County, Washington. It was built in 1940 and reconstructed in 1970, so it is 85 years old, older than the Washington average of 49.8 years.
The most recent inspection reported to the Federal Highway Administration rates it in poor condition with a lowest component rating of 4 out of 9. The lowest-rated primary component is rated 4 or below, indicating advanced deterioration. Poor does not mean unsafe; such bridges are inspected more often and may carry load restrictions. Its operating status is: posted for load restriction.
It carries about 74,410 vehicles a day, roughly 3% of them trucks, projected to reach 117,268 by 2039. The structure is 2,812 ft (857.1 m) long and 65 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 4 or below, indicating advanced deterioration. Poor does not mean unsafe; such bridges are inspected more often and may carry load restrictions.
| Component | Rating | Meaning |
|---|---|---|
| Deck | 6 Satisfactory | Structural elements show some minor deterioration. |
| Superstructure | 4 Poor | Advanced section loss, deterioration, 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 1940, making it 85 years old in 2025. It was reconstructed in 1970.
What condition is I-90 over Mercer Slough in?
The National Bridge Inventory rates it in poor condition. The lowest-rated primary component is rated 4 or below, indicating advanced deterioration. Poor does not mean unsafe; such bridges are inspected more often and may carry load restrictions. The last reported inspection was in February 2023.
How much traffic does I-90 over Mercer Slough carry?
About 74,410 vehicles per day (2023 count), of which roughly 3% are trucks.
How long is I-90 over Mercer Slough?
The structure is 2,812 ft (857.1 m) long with a longest span of 95 ft (29 m) and a deck 65 ft wide.
Who owns I-90 over Mercer Slough?
It is owned by the State Highway Agency.