US 18 over Big Sioux River
Lyon County, Iowa · rated Fair in the 2025 National Bridge Inventory
United States Bridge Conditions2025 National Bridge Inventory, FHWACompiled
- US 18 over Big Sioux River in Lyon County, Iowa, is rated in fair condition in the 2025 National Bridge Inventory (as of ).
- Its lowest-rated component is the deck, rated 5 (fair condition) on the 0-9 scale.
- It was built in 1978, making it 47 years old at the 2025 inventory.
- It carries about 2,130 vehicles a day (traffic count year 2022).
- Its listed owner is the state highway agency.
- It is one of 99 of the 273 inventoried bridges in Lyon County rated fair (36.3%); 70 (25.6%) are rated poor.
- Its traffic status in the 2025 inventory is open, no restriction (NBI item 41).
- Its class is unchanged from 2024 (fair), but component ratings moved: deck 6 to 5.
- Its most recent inspection on record is dated March 2024, 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 → (43.30718, -96.52828)
Component ratings
| Component | Rating (2024) | Rating (2025) | FHWA scale meaning |
|---|---|---|---|
| Deck | 6 | 5 | Fair condition |
| Superstructure | 6 | 6 | Satisfactory condition |
| Substructure | 7 | 7 | Good condition |
Bridge details
| Structure | Continuous steel stringer or multi-beam girder |
| Length | about 580 ft |
| Location | at S.DAKOTA BORDER |
| NBI structure number | 000000000034481 |
| Condition class | Fair |
| Traffic status (item 41) | Open, no restriction |
| Last inspection (item 90) | March 2024 |
| Inspection interval (item 91) | 24 months |
| Scour (item 113) | 8: foundations stable, scour above the top of the footing |
Source: FHWA National Bridge Inventory, 2025 data as reported by the Iowa department of transportation.
Busiest bridges in Lyon County
| Bridge | Built | Vehicles/day | Condition |
|---|---|---|---|
| Ia 9 over Stream | 1980 | 4,840 | Good |
| Ia 9 over Stream | 1940 | 4,520 | Fair |
| Ia 9 over Bloodrun Creek | 1977 | 4,520 | Fair |
| US 75 & Ia 9 over Moon Creek | 1972 | 4,400 | Fair |
| Ia 9 over Rock River | 1974 | 3,920 | Fair |
| Ia 9 over Mud Creek | 1975 | 3,270 | Fair |
All bridges nearby: Lyon County, IA bridge conditions · every bridge in Lyon County · Iowa bridges.
Background: how bridge ratings work · what "poor" and "structurally deficient" actually mean · how often bridges are inspected.