US Bridges

US 20 over Knowlton Creek

Fair

Camas County, Idaho · 3.4 E. Fairfield · Structure no. 000000000015065

Built
1958
67 years old
Condition
Fair
inspected August 2023
Type
Stringer/Multi-beam or Girder
Concrete
Length
30 ft
1 main span
Daily traffic
1,900
2023 count
Owner
State
State Highway Agency

About this bridge

US 20 over Knowlton Creek is a concrete stringer/multi-beam or girder carrying US 20 over Knowlton Creek in Camas County, Idaho. It was built in 1958, so it is 67 years old, older than the Idaho average of 45.3 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 1,900 vehicles a day, roughly 25% of them trucks, projected to reach 2,850 by 2042. The structure is 30 ft (9.1 m) long and 31 ft wide. If it were closed, the detour would be 98.8 mi (159 km).

Location

Open in Google Maps
CarriesUS 20
CrossesKnowlton Creek
Location3.4 E. Fairfield
RouteU.S. Route 20
CountyCamas County
StateIdaho
Coordinates43.34223, -114.72373
Functional classRural Principal Arterial - Other

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 protection5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Last inspectionAugust 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourScour critical; foundations unstable for calculated scour conditions
Structural evaluation6: Equal to present minimum criteria
Deck geometry5: Somewhat better than minimum adequacy to tolerate being left in place as is
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating71 US tons
Inventory rating43 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialConcrete
DeckConcrete, cast-in-place
Main spans1
Design loadHS 20
Structure length30 ft (9.1 m)
Longest span29 ft (8.8 m)
Deck width31 ft (9.3 m)
Roadway width28 ft (8.5 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic1,900 (2023)
Projected traffic2,850 by 2042
Trucks25%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed98.8 mi (159 km)
TollOn a free road
National Highway SystemYes

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusHistorical significance not determinable at this time
Work proposedReplacement because of substandard load-carrying capacity or roadway geometry
Estimated improvement cost$490,000 (2023 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was US 20 over Knowlton Creek built?

It was built in 1958, making it 67 years old in 2025.

What condition is US 20 over Knowlton Creek 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 US 20 over Knowlton Creek carry?

About 1,900 vehicles per day (2023 count), of which roughly 25% are trucks.

How long is US 20 over Knowlton Creek?

The structure is 30 ft (9.1 m) long with a longest span of 29 ft (8.8 m) and a deck 31 ft wide.

Who owns US 20 over Knowlton Creek?

It is owned by the State Highway Agency.