US Bridges

4100 W over Big Lost River

Fair

Custer County, Idaho · 2.5 S. 5.2 E. Mackay · Structure no. 000000000031710

Built
1977
48 years old
Condition
Fair
inspected October 2023
Type
Tee Beam
Prestressed Concrete
Length
70 ft
1 main span
Daily traffic
50
2023 count
Owner
Local
Other Local Agency

About this bridge

4100 W over Big Lost River is a prestressed concrete tee beam carrying 4100 W over Big Lost River in Custer County, Idaho. It was built in 1977, so it is 48 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 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 50 vehicles a day, roughly 10% of them trucks, projected to reach 75 by 2039. The structure is 70 ft (21.3 m) long and 28 ft wide. If it were closed, the detour would be 5 mi (8 km).

Location

Open in Google Maps
Carries4100 W
CrossesBig Lost River
Location2.5 S. 5.2 E. Mackay
RouteCounty Highway 0
CountyCuster County
StateIdaho
Coordinates43.87036, -113.51256
Functional classRural Local

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Deck7 GoodSome minor problems.
Superstructure7 GoodSome minor problems.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Channel and channel protection4 PoorAdvanced section loss, deterioration, spalling or scour.
Last inspectionOctober 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; action required to protect exposed foundations
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry6: Equal to present minimum criteria
Waterway adequacy9: Superior to present desirable criteria
Approach roadway alignment6: Equal to present minimum criteria
Operating rating62.5 US tons
Inventory rating45.6 US tons

Structure

Structure typeTee Beam
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans1
Design loadHS 20
Structure length70 ft (21.3 m)
Longest span68 ft (20.7 m)
Deck width28 ft (8.5 m)
Roadway width25 ft (7.7 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic50 (2023)
Projected traffic75 by 2039
Trucks10%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed5 mi (8 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerOther Local Agency
Maintained byOther Local Agency
Historic statusHistorical significance not determinable at this time
Work proposedNone recorded
Estimated improvement cost—
NavigationNo navigation control on waterway

Frequently asked questions

When was 4100 W over Big Lost River built?

It was built in 1977, making it 48 years old in 2025.

What condition is 4100 W over Big Lost River 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 October 2023.

How much traffic does 4100 W over Big Lost River carry?

About 50 vehicles per day (2023 count), of which roughly 10% are trucks.

How long is 4100 W over Big Lost River?

The structure is 70 ft (21.3 m) long with a longest span of 68 ft (20.7 m) and a deck 28 ft wide.

Who owns 4100 W over Big Lost River?

It is owned by the Other Local Agency.