M-21 over Shiawassee River
Shiawassee County, Michigan · rated Fair in the 2025 National Bridge Inventory
United States Bridge Conditions2025 National Bridge Inventory, FHWACompiled
- M-21 over Shiawassee River in Shiawassee County, Michigan, is rated in fair condition in the 2025 National Bridge Inventory (as of ).
- Its lowest-rated component is the deck, rated 6 (satisfactory condition) on the 0-9 scale.
- It was built in 1970, making it 55 years old at the 2025 inventory.
- It carries about 22,689 vehicles a day (traffic count year 2007).
- Its listed owner is the state highway agency.
- It is one of 76 of the 163 inventoried bridges in Shiawassee County rated fair (46.6%); 15 (9.2%) 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 July 2023, on a 24-month routine inspection cycle.
- It is coded scour critical (NBI item 113 = 3): scour critical: foundations unstable for the assessed or calculated scour.
- 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 → (42.99756, -84.17384)
Component ratings
| Component | Rating (2024) | Rating (2025) | FHWA scale meaning |
|---|---|---|---|
| Deck | 6 | 6 | Satisfactory condition |
| Superstructure | 6 | 6 | Satisfactory condition |
| Substructure | 6 | 6 | Satisfactory condition |
Bridge details
| Structure | Steel stringer or multi-beam girder |
| Length | about 131 ft |
| Location | In Owosso |
| NBI structure number | 000000000009773 |
| Condition class | Fair |
| Traffic status (item 41) | Open, no restriction |
| Last inspection (item 90) | July 2023 |
| Inspection interval (item 91) | 24 months |
| Scour (item 113) | 3: scour critical: foundations unstable for the assessed or calculated scour |
Source: FHWA National Bridge Inventory, 2025 data as reported by the Michigan department of transportation.
Busiest bridges in Shiawassee County
| Bridge | Built | Vehicles/day | Condition |
|---|---|---|---|
| I-69 EB over Looking Glass River | 1961 | 192,000 | Fair |
| I-69 WB over Looking Glass River | 1961 | 192,000 | Fair |
| I-69 EB over Holly Drain | 1961 | 192,000 | Fair |
| I-69 WB over Holly Drain | 1961 | 192,000 | Fair |
| Britton Rd over I-69 | 1989 | 25,000 | Fair |
| I-69 EB over Vermillion Creek | 1989 | 20,138 | Fair |
All bridges nearby: Shiawassee County, MI bridge conditions · every bridge in Shiawassee County · Michigan bridges.
Background: how bridge ratings work · what "poor" and "structurally deficient" actually mean · how often bridges are inspected.