US Bridges

I-90 over SR 18

Fair

King County, Washington · 9.7 E JCT SR 900 · Structure no. 0006966B0000000

Built
1963
62 years old, rebuilt 1978
Condition
Fair
inspected June 2023
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
158 ft
3 main spans
Daily traffic
22,901
2023 count
Owner
State
State Highway Agency

About this bridge

I-90 over SR 18 is a prestressed concrete stringer/multi-beam or girder carrying I-90 over SR 18 in King County, Washington. It was built in 1963 and reconstructed in 1978, so it is 62 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 22,901 vehicles a day, roughly 7% of them trucks, projected to reach 32,154 by 2039. The structure is 158 ft (48.2 m) long and 75 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesI-90
CrossesSR 18
Location9.7 E JCT SR 900
RouteInterstate 90
CountyKing County
StateWashington
Coordinates47.509, -121.88392
Functional classRural 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.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure7 GoodSome minor problems.
Last inspectionJune 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourBridge not over waterway
Structural evaluation6: Equal to present minimum criteria
Deck geometry9: Superior to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating60 US tons
Inventory rating36 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans3
Design loadHS 20
Structure length158 ft (48.2 m)
Longest span72 ft (21.9 m)
Deck width75 ft (23 m)
Roadway width70 ft (21.3 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic22,901 (2023)
Projected traffic32,154 by 2039
Trucks7%
Lanes on structure3
Traffic directionOne-way traffic
CarriesHighway
CrossesHighway
Detour if closed1.2 mi (2 km)
TollOn a free road
National Highway SystemYes
Strategic Highway NetworkInterstate STRAHNET route

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$5.4 million (2022 estimate)
NavigationNot applicable (no waterway)

Frequently asked questions

When was I-90 over SR 18 built?

It was built in 1963, making it 62 years old in 2025. It was reconstructed in 1978.

What condition is I-90 over SR 18 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 June 2023.

How much traffic does I-90 over SR 18 carry?

About 22,901 vehicles per day (2023 count), of which roughly 7% are trucks.

How long is I-90 over SR 18?

The structure is 158 ft (48.2 m) long with a longest span of 72 ft (21.9 m) and a deck 75 ft wide.

Who owns I-90 over SR 18?

It is owned by the State Highway Agency.