US Bridges

SR 20 over Swinomish Channel

Good

Skagit County, Washington · 9.3 E Island CO · Structure no. 0011612A0000000

Built
1981
44 years old
Condition
Good
inspected January 2023
Type
Stringer/Multi-beam or Girder
Steel Continuous
Length
3,545 ft
5 main spans
Daily traffic
18,383
2023 count
Owner
State
State Highway Agency

About this bridge

SR 20 over Swinomish Channel is a steel continuous stringer/multi-beam or girder carrying SR 20 over Swinomish Channel in Skagit County, Washington. It was built in 1981, so it is 44 years old, younger than the Washington average of 49.8 years.

The most recent inspection reported to the Federal Highway Administration rates it in good condition with a lowest component rating of 7 out of 9. All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted.

It carries about 18,383 vehicles a day, roughly 6% of them trucks, projected to reach 25,365 by 2039. The structure is 3,545 ft (1,080.5 m) long and 42 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesSR 20
CrossesSwinomish Channel
Location9.3 E Island CO
RouteState Highway 20
CountySkagit County
StateWashington
Coordinates48.45636, -122.52186
Functional classUrban Principal Arterial - Other Freeway or Expressway

Condition and inspection

Good

All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted.

ComponentRatingMeaning
Deck7 GoodSome minor problems.
Superstructure7 GoodSome minor problems.
Substructure7 GoodSome minor problems.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionJanuary 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour within limits of footing or piles
Structural evaluation7: Better than 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 rating63.9 US tons
Inventory rating38 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel Continuous
DeckConcrete, cast-in-place
Main spans5 (+19 approach)
Design loadHS 20 + Mod
Structure length3,545 ft (1,080.5 m)
Longest span246 ft (75 m)
Deck width42 ft (12.8 m)
Roadway width33 ft (10.1 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic18,383 (2023)
Projected traffic25,365 by 2039
Trucks6%
Lanes on structure2
Traffic directionOne-way traffic
CarriesHighway
CrossesHighway and waterway
Detour if closed1.2 mi (2 km)
TollOn a free road
National Highway SystemYes
Strategic Highway NetworkSTRAHNET connector

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$58.9 million (2022 estimate)
NavigationNavigation control on waterway (bridge permit required)

Frequently asked questions

When was SR 20 over Swinomish Channel built?

It was built in 1981, making it 44 years old in 2025.

What condition is SR 20 over Swinomish Channel in?

The National Bridge Inventory rates it in good condition. All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted. The last reported inspection was in January 2023.

How much traffic does SR 20 over Swinomish Channel carry?

About 18,383 vehicles per day (2023 count), of which roughly 6% are trucks.

How long is SR 20 over Swinomish Channel?

The structure is 3,545 ft (1,080.5 m) long with a longest span of 246 ft (75 m) and a deck 42 ft wide.

Who owns SR 20 over Swinomish Channel?

It is owned by the State Highway Agency.