US Bridges

I-90 over Mercer Slough

Fair

King County, Washington · 6.8 E JCT I-5 · Structure no. 0008610F0000000

Built
1970
55 years old
Condition
Fair
inspected February 2023
Type
Box Beam or Girders - Multiple
Concrete Continuous
Length
970 ft
8 main spans
Daily traffic
3,203
2010 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 1970, 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 6 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 3,203 vehicles a day, roughly 5% of them trucks, projected to reach 5,535 by 2036. The structure is 970 ft (295.7 m) long and 26 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesI-90
CrossesMercer Slough
Location6.8 E JCT I-5
RouteInterstate 90
CountyKing County
StateWashington
Coordinates47.57878, -122.18628
Functional classUrban Principal Arterial - Interstate

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Deck7 GoodSome minor problems.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure7 GoodSome minor problems.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionFebruary 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry6: Equal to present minimum criteria
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating28.5 US tons
Inventory rating21.8 US tons

Structure

Structure typeBox Beam or Girders - Multiple
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans8 (+8 approach)
Design loadHS 20
Structure length970 ft (295.7 m)
Longest span84 ft (25.6 m)
Deck width26 ft (7.8 m)
Roadway width23 ft (7 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic3,203 (2010)
Projected traffic5,535 by 2036
Trucks5%
Lanes on structure1
Traffic directionOne-way traffic
CarriesHighway
CrossesWaterway
Detour if closed1.2 mi (2 km)
TollOn a free road
National Highway SystemYes

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusNot eligible for the National Register of Historic Places
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$12.4 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was I-90 over Mercer Slough built?

It was built in 1970, making it 55 years old in 2025.

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 3,203 vehicles per day (2010 count), of which roughly 5% are trucks.

How long is I-90 over Mercer Slough?

The structure is 970 ft (295.7 m) long with a longest span of 84 ft (25.6 m) and a deck 26 ft wide.

Who owns I-90 over Mercer Slough?

It is owned by the State Highway Agency.