US Bridges

I-90 over Mercer Slough

Fair

King County, Washington · 6.4 E JCT I-5 · Structure no. 0008610D0000000

Built
1970
55 years old
Condition
Fair
inspected February 2023
Type
Slab
Concrete Continuous
Length
1,871 ft
44 main spans
Daily traffic
22,361
2023 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, 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 22,361 vehicles a day, roughly 5% of them trucks, projected to reach 34,283 by 2039. The structure is 1,871 ft (570.3 m) long and 36 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesI-90
CrossesMercer Slough
Location6.4 E JCT I-5
RouteInterstate 90
CountyKing County
StateWashington
Coordinates47.57978, -122.18631
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
Deck6 SatisfactoryStructural elements show some minor deterioration.
Superstructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
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 evaluation4: Meets minimum tolerable limits to be left in place as is
Deck geometry2: Basically intolerable, requiring high priority of replacement
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating26.1 US tons
Inventory rating20 US tons

Structure

Structure typeSlab
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans44
Design loadHS 20 + Mod
Structure length1,871 ft (570.3 m)
Longest span56 ft (17.1 m)
Deck width36 ft (11.1 m)
Roadway width33 ft (10.1 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic22,361 (2023)
Projected traffic34,283 by 2039
Trucks5%
Lanes on structure3
Traffic directionOne-way traffic
CarriesOverpass structure at an interchange
CrossesHighway and waterway
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$33.8 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 22,361 vehicles per day (2023 count), of which roughly 5% are trucks.

How long is I-90 over Mercer Slough?

The structure is 1,871 ft (570.3 m) long with a longest span of 56 ft (17.1 m) and a deck 36 ft wide.

Who owns I-90 over Mercer Slough?

It is owned by the State Highway Agency.