US Bridges

SR 99 over Lake Union

Fair

King County, Washington · 7.7 N JCT SR 509 · Structure no. 0001447A0000000

Built
1931
94 years old
Condition
Fair
inspected August 2023
Type
Truss - Deck
Steel
Length
2,955 ft
8 main spans
Daily traffic
61,259
2023 count
Owner
State
State Highway Agency

About this bridge

SR 99 over Lake Union is a steel truss - deck carrying SR 99 over Lake Union in King County, Washington. It was built in 1931, so it is 94 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 61,259 vehicles a day, roughly 4% of them trucks, projected to reach 100,765 by 2039. The structure is 2,955 ft (900.7 m) long and 70 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesSR 99
CrossesLake Union
Location7.7 N JCT SR 509
RouteState Highway 99
CountyKing County
StateWashington
Coordinates47.64318, -122.34719
Functional classUrban Principal Arterial - Other Freeway or Expressway

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 inspectionAugust 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation3: Basically intolerable, requiring high priority of corrective action
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 rating23 US tons
Inventory rating14 US tons

Structure

Structure typeTruss - Deck
MaterialSteel
DeckConcrete, cast-in-place
Main spans8 (+15 approach)
Design loadH 20
Structure length2,955 ft (900.7 m)
Longest span800 ft (243.8 m)
Deck width70 ft (21.2 m)
Roadway width56 ft (17.2 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic61,259 (2023)
Projected traffic100,765 by 2039
Trucks4%
Lanes on structure6
Traffic directionTwo-way traffic
CarriesHighway and pedestrian
CrossesHighway, waterway and railroad
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 statusListed on the National Register of Historic Places
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$82.8 million (2022 estimate)
NavigationNavigation control on waterway (bridge permit required)

Frequently asked questions

When was SR 99 over Lake Union built?

It was built in 1931, making it 94 years old in 2025.

What condition is SR 99 over Lake Union 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 August 2023.

How much traffic does SR 99 over Lake Union carry?

About 61,259 vehicles per day (2023 count), of which roughly 4% are trucks.

How long is SR 99 over Lake Union?

The structure is 2,955 ft (900.7 m) long with a longest span of 800 ft (243.8 m) and a deck 70 ft wide.

Who owns SR 99 over Lake Union?

It is owned by the State Highway Agency.