US Bridges

I-90 S-E Ramp over I-90 Mercer Slough

Fair

King County, Washington · 6.4 E JCT I-5 · Structure no. 0008610E0000000

Built
1970
55 years old
Condition
Fair
inspected February 2023
Type
Box Beam or Girders - Multiple
Concrete Continuous
Length
1,929 ft
20 main spans
Daily traffic
8,091
2012 count
Owner
State
State Highway Agency

About this bridge

I-90 S-E Ramp over I-90 Mercer Slough is a concrete continuous box beam or girders - multiple carrying I-90 S-E Ramp over I-90 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 8,091 vehicles a day, roughly 5% of them trucks, projected to reach 13,528 by 2036. The structure is 1,929 ft (588 m) long and 28 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesI-90 S-E Ramp
CrossesI-90 Mercer Slough
Location6.4 E JCT I-5
RouteInterstate 90
CountyKing County
StateWashington
Coordinates47.58001, -122.18904
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
Deck5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
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 within limits of footing or piles
Structural evaluation3: Basically intolerable, requiring high priority of corrective action
Deck geometry7: Better than present minimum criteria
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating37.8 US tons
Inventory rating9.3 US tons

Structure

Structure typeBox Beam or Girders - Multiple
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans20 (+4 approach)
Design loadHS 20
Structure length1,929 ft (588 m)
Longest span152 ft (46.3 m)
Deck width28 ft (8.4 m)
Roadway width24 ft (7.3 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic8,091 (2012)
Projected traffic13,528 by 2036
Trucks5%
Lanes on structure1
Traffic directionOne-way traffic
CarriesThird level (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$27.5 million (2022 estimate)
NavigationNavigation control on waterway (bridge permit required)

Frequently asked questions

When was I-90 S-E Ramp over I-90 Mercer Slough built?

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

What condition is I-90 S-E Ramp over I-90 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 S-E Ramp over I-90 Mercer Slough carry?

About 8,091 vehicles per day (2012 count), of which roughly 5% are trucks.

How long is I-90 S-E Ramp over I-90 Mercer Slough?

The structure is 1,929 ft (588 m) long with a longest span of 152 ft (46.3 m) and a deck 28 ft wide.

Who owns I-90 S-E Ramp over I-90 Mercer Slough?

It is owned by the State Highway Agency.