US Bridges

I-5 over Union Slough

Fair

Snohomish County, Washington · 3.3 N JCT US 2 · Structure no. 0008226C0000000

Built
1968
57 years old
Condition
Fair
inspected June 2023
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
396 ft
4 main spans
Daily traffic
64,989
2023 count
Owner
State
State Highway Agency

About this bridge

I-5 over Union Slough is a prestressed concrete stringer/multi-beam or girder carrying I-5 over Union Slough in Snohomish County, Washington. It was built in 1968, so it is 57 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 64,989 vehicles a day, roughly 6% of them trucks, projected to reach 95,649 by 2039. The structure is 396 ft (120.7 m) long and 52 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesI-5
CrossesUnion Slough
Location3.3 N JCT US 2
RouteInterstate 5
CountySnohomish County
StateWashington
Coordinates48.02314, -122.17578
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.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionJune 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry4: Meets minimum tolerable limits to be left in place as is
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating75 US tons
Inventory rating41 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans4 (+2 approach)
Design loadHS 20 + Mod
Structure length396 ft (120.7 m)
Longest span86 ft (26.2 m)
Deck width52 ft (15.7 m)
Roadway width48 ft (14.6 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic64,989 (2023)
Projected traffic95,649 by 2039
Trucks6%
Lanes on structure3
Traffic directionOne-way traffic
CarriesHighway
CrossesWaterway
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 statusHistorical significance not determinable at this time
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$10.5 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was I-5 over Union Slough built?

It was built in 1968, making it 57 years old in 2025.

What condition is I-5 over Union 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 June 2023.

How much traffic does I-5 over Union Slough carry?

About 64,989 vehicles per day (2023 count), of which roughly 6% are trucks.

How long is I-5 over Union Slough?

The structure is 396 ft (120.7 m) long with a longest span of 86 ft (26.2 m) and a deck 52 ft wide.

Who owns I-5 over Union Slough?

It is owned by the State Highway Agency.