I-75 SB over Portage St
Mackinac County, Michigan · rated Fair in the 2025 National Bridge Inventory
United States Bridge Conditions2025 National Bridge Inventory, FHWACompiled
- I-75 SB over Portage St in Mackinac County, Michigan, is rated in fair condition in the 2025 National Bridge Inventory (as of ).
- Its lowest-rated component is the substructure, rated 5 (fair condition) on the 0-9 scale.
- It was built in 1960, making it 65 years old at the 2025 inventory.
- It carries about 2,555 vehicles a day (traffic count year 2007).
- Its listed owner is the state highway agency.
- It is one of 36 of the 63 inventoried bridges in Mackinac County rated fair (57.1%); 10 (15.9%) are rated poor.
- Its traffic status in the 2025 inventory is open, no restriction (NBI item 41).
- Its condition class and component ratings are unchanged from the 2024 inventory.
- Its most recent inspection on record is dated April 2023, on a 24-month routine inspection cycle.
- A fair rating means the lowest-rated major component scores 5 or 6 on FHWA's 0-9 scale: some deterioration or minor section loss, but the structure is sound. It is the middle class, not a warning.
View this bridge on a map → (45.86664, -84.73486)
Component ratings
| Component | Rating (2024) | Rating (2025) | FHWA scale meaning |
|---|---|---|---|
| Deck | 6 | 6 | Satisfactory condition |
| Superstructure | 6 | 6 | Satisfactory condition |
| Substructure | 5 | 5 | Fair condition |
Bridge details
| Structure | Steel stringer or multi-beam girder |
| Length | about 137 ft |
| Location | In St Ignace Portage St |
| NBI structure number | 000000000005951 |
| Condition class | Fair |
| Traffic status (item 41) | Open, no restriction |
| Last inspection (item 90) | April 2023 |
| Inspection interval (item 91) | 24 months |
| Scour (item 113) | N: not over a waterway |
Source: FHWA National Bridge Inventory, 2025 data as reported by the Michigan department of transportation.
Busiest bridges in Mackinac County
| Bridge | Built | Vehicles/day | Condition |
|---|---|---|---|
| Us-2|i-75 Bl over I-75 NB | 1957 | 7,550 | Fair |
| US-2 over Cut River | 1947 | 4,450 | Fair |
| US-2 over Brevort River | 2019 | 4,400 | Good |
| I-75 SB over Carp River | 1964 | 4,400 | Fair |
| I-75 SB over Mackinac Trail | 1963 | 4,400 | Fair |
| I-75 Bl over I-75 NB | 1960 | 4,350 | Fair |
All bridges nearby: Mackinac County, MI bridge conditions · every bridge in Mackinac County · Michigan bridges.
Background: how bridge ratings work · what "poor" and "structurally deficient" actually mean · how often bridges are inspected.