US Bridges

I-75 over Straits of Mackinac

FairPosted for load

Emmet County, Michigan · Mackinaw City-St Ignace · Structure no. 000000000002456

Built
1957
68 years old
Condition
Fair
inspected July 2024
Type
Suspension
Steel
Length
19,248 ft
3 main spans
Daily traffic
11,926
2021 count
Owner
Toll Authority
State Toll Authority

About this bridge

I-75 over Straits of Mackinac is a steel suspension carrying I-75 over Straits of Mackinac in Emmet County, Michigan. It was built in 1957, so it is 68 years old, older than the Michigan average of 48.3 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. Its operating status is: posted for load restriction.

It carries about 11,926 vehicles a day, roughly 7% of them trucks, projected to reach 13,715 by 2041. The structure is 19,248 ft (5,866.7 m) long and 54 ft wide. If it were closed, the detour would be 124.3 mi (200 km).

Location

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CarriesI-75
CrossesStraits of Mackinac
LocationMackinaw City-St Ignace
RouteInterstate 75
CountyEmmet County
StateMichigan
Coordinates45.83684, -84.72481
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.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure7 GoodSome minor problems.
Channel and channel protection9 ExcellentNo problems noted.
Last inspectionJuly 2024
Inspection intervalevery 12 months
Operating statusPosted for load restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry2: Basically intolerable, requiring high priority of replacement
Waterway adequacy9: Superior to present desirable criteria
Approach roadway alignment9: Superior to present desirable criteria
Operating rating52.9 US tons
Inventory rating36.8 US tons

Structure

Structure typeSuspension
MaterialSteel
DeckConcrete, cast-in-place
Main spans3 (+57 approach)
Design loadHS 20
Structure length19,248 ft (5,866.7 m)
Longest span3,801 ft (1,158.5 m)
Deck width54 ft (16.4 m)
Roadway width48 ft (14.6 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic11,926 (2021)
Projected traffic13,715 by 2041
Trucks7%
Lanes on structure4
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed124.3 mi (200 km)
TollToll bridge
National Highway SystemYes
Strategic Highway NetworkInterstate STRAHNET route

Ownership and planned work

OwnerState Toll Authority
Maintained byState Toll Authority
Historic statusListed on the National Register of Historic Places
Work proposedNone recorded
Estimated improvement cost—
NavigationNavigation control on waterway (bridge permit required)

Frequently asked questions

When was I-75 over Straits of Mackinac built?

It was built in 1957, making it 68 years old in 2025.

What condition is I-75 over Straits of Mackinac 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 2024.

How much traffic does I-75 over Straits of Mackinac carry?

About 11,926 vehicles per day (2021 count), of which roughly 7% are trucks.

How long is I-75 over Straits of Mackinac?

The structure is 19,248 ft (5,866.7 m) long with a longest span of 3,801 ft (1,158.5 m) and a deck 54 ft wide.

Who owns I-75 over Straits of Mackinac?

It is owned by the State Toll Authority.