US Bridges

I 15 over Boulder River

Fair

Jefferson County, Montana · 1.2M S Basin · Structure no. I00015155+09441

Built
1985
40 years old
Condition
Fair
inspected July 2023
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
108 ft
1 main span
Daily traffic
3,182
2023 count
Owner
State
State Highway Agency

About this bridge

I 15 over Boulder River is a prestressed concrete stringer/multi-beam or girder carrying I 15 over Boulder River in Jefferson County, Montana. It was built in 1985, so it is 40 years old, younger than the Montana average of 47 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 3,182 vehicles a day, roughly 16% of them trucks, projected to reach 3,437 by 2043. The structure is 108 ft (32.9 m) long and 84 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesI 15
CrossesBoulder River
Location1.2M S Basin
RouteInterstate 15
CountyJefferson County
StateMontana
Coordinates46.26979, -112.26615
Functional classRural 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.
Superstructure7 GoodSome minor problems.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionJuly 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry9: Superior to present desirable criteria
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating64 US tons
Inventory rating50 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans1
Design loadHS 20
Structure length108 ft (32.9 m)
Longest span105 ft (32 m)
Deck width84 ft (25.7 m)
Roadway width81 ft (24.8 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic3,182 (2023)
Projected traffic3,437 by 2043
Trucks16%
Lanes on structure4
Traffic directionTwo-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 statusNot eligible for the National Register of Historic Places
Work proposedNone recorded
Estimated improvement cost—
NavigationNo navigation control on waterway

Frequently asked questions

When was I 15 over Boulder River built?

It was built in 1985, making it 40 years old in 2025.

What condition is I 15 over Boulder 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 July 2023.

How much traffic does I 15 over Boulder River carry?

About 3,182 vehicles per day (2023 count), of which roughly 16% are trucks.

How long is I 15 over Boulder River?

The structure is 108 ft (32.9 m) long with a longest span of 105 ft (32 m) and a deck 84 ft wide.

Who owns I 15 over Boulder River?

It is owned by the State Highway Agency.