I-75
over STRAITS OF MACKINAC
MACKINAW CITY-ST IGNACE, Emmet County, Michigan
Condition
GoodYear Built
1957
Daily Traffic
11,076
Length
1,925 ft
About This Bridge
I-75 carries traffic over STRAITS OF MACKINAC in Emmet County, Michigan. Built in 1957, this structure is 69 years old, placing it among the older bridges still in active service. The bridge features steel construction with a suspension design, a combination chosen by engineers to meet the specific requirements of this crossing.
Approximately 11,076 vehicles cross this bridge daily, representing moderate traffic volumes typical of important regional connectors. This level of usage requires regular monitoring to ensure continued safe operation. Commercial trucks account for 7% of all crossings. This proportion of commercial traffic is typical for this roadway classification and is factored into the bridge's design load ratings.
The bridge is currently rated in good condition based on federal inspection criteria established by the Federal Highway Administration. The structure meets current safety standards for its designated use and load capacity. With a total length of 1,925 feet, this is a major span requiring specialized engineering expertise for inspection and maintenance. Longer bridges present unique challenges including thermal expansion, wind loads, and more complex structural analysis.
Steel bridges provide high strength-to-weight ratios, making them suitable for longer spans. However, they require ongoing protection against corrosion through painting or weathering steel designs. Inspectors pay close attention to connection points, fatigue cracking, and coating condition.
Federal law requires that all highway bridges be inspected at least every 24 months by qualified inspectors following the National Bridge Inspection Standards (NBIS). Some bridges with known issues or unusual characteristics may be inspected more frequently. Inspection data is reported to the National Bridge Inventory, a database maintained by the Federal Highway Administration that tracks the condition of all public highway bridges in the United States. This bridge data is sourced from that federal database and reflects the most recent available inspection results.
Condition Ratings
Deck Condition
Good
Superstructure
Good
Substructure
Very Good
Channel & Protection
Excellent
Lowest Rating
Determines condition category
NBI condition ratings range from 0 (failed) to 9 (excellent). Ratings of 4 or below are considered poor. The overall condition category is determined by the lowest individual component rating.
With a lowest rating of 7, this bridge demonstrates good condition meeting all safety standards.
For a structure built in 1957, maintaining good condition demonstrates effective maintenance practices. If this bridge were closed, traffic would face a detour of approximately 124 miles, underscoring the importance of maintaining this crossing.
Bridge Details
Steel is the primary structural material, which influences maintenance requirements and expected lifespan. The suspension design was selected based on span requirements, load capacity needs, and site conditions at the time of construction. Maintenance responsibility falls to State Toll Authority.
Construction
Dimensions
The maximum span length indicates this is a major crossing requiring specialized engineering. The deck area of 96,214 sq ft determines surface maintenance scope.
Classification
Status
Traffic Data
Traffic volume data helps engineers assess bridge wear patterns and plan maintenance schedules. Average Daily Traffic (ADT) counts are updated periodically and represent typical weekday usage.
11,076
Avg Daily Traffic (2007)
Vehicles per day crossing this bridge
7%
Truck Traffic
Commercial vehicle percentage
17,233
Projected Future ADT
Estimated future demand
This bridge carries approximately 11,076 vehicles per day, translating to roughly 4,042,740 crossings annually. This moderate traffic level is typical for regional connectors.
Commercial trucks comprise 7% of traffic. Heavy vehicles cause disproportionate wear—a single fully loaded truck can have the impact of thousands of passenger cars on bridge deck surfaces.
Traffic projections estimate future volumes of 17,233 vehicles daily, a 56% increase from current levels. These projections inform capacity planning and potential upgrade requirements.
Load Ratings
Load ratings determine what weight this bridge can safely support. These values are critical for routing heavy vehicles and issuing overweight permits.
Operating Rating
4.8 tons
Maximum allowable load for special permits
Inventory Rating
3.3 tons
Load level for normal traffic
The operating rating of 4.8 tons represents the absolute maximum load this bridge can carry under controlled conditions with special permits. The inventory rating of 3.3 tons is the safe load limit for everyday traffic without restrictions. These relatively low ratings may result in posted weight limits or route restrictions for heavy vehicles.
5-Year Condition Trend
↓DecliningCondition has declined by 1.0 rating points since 2020
Data: 2020-2024 FHWA National Bridge Inventory
Location
Loading map...
46.183889, -85.530556
Identification
Explore More
Data Source: Federal Highway Administration (FHWA) National Bridge Inventory 2024
Bridge inspection data is typically updated every 24 months. Conditions may have changed since the last inspection.
“Structurally deficient” does not mean a bridge is unsafe or likely to collapse. It indicates that one or more key structural elements are in poor or worse condition. Bridges are inspected regularly and may have load restrictions in place.
This data is for informational purposes only and should not be used for route clearance or vehicle weight decisions.