State Route 255 over Mad River Slough
Humboldt County, California · rated Good in the 2025 National Bridge Inventory
United States Bridge Conditions2025 National Bridge Inventory, FHWACompiled
- State Route 255 over Mad River Slough in Humboldt County, California, is rated in good condition in the 2025 National Bridge Inventory (as of ).
- Its lowest-rated component is the deck, rated 7 (good condition) on the 0-9 scale.
- It was built in 1981, making it 44 years old at the 2025 inventory.
- It carries about 4,300 vehicles a day (traffic count year 2010).
- Its listed owner is the state highway agency.
- It is one of 169 of the 399 inventoried bridges in Humboldt County rated good (42.4%); 21 (5.3%) 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 September 2023, on a 48-month routine inspection cycle.
- A good rating means every major component - deck, superstructure, substructure, or culvert - is rated 7 or higher on FHWA's 0-9 scale: no more than minor problems.
View this bridge on a map → (40.86497, -124.1505)
Component ratings
| Component | Rating (2024) | Rating (2025) | FHWA scale meaning |
|---|---|---|---|
| Deck | 7 | 7 | Good condition |
| Superstructure | 7 | 7 | Good condition |
| Substructure | 7 | 7 | Good condition |
Bridge details
| Structure | Continuous prestressed concrete stringer or multi-beam girder |
| Length | about 497 ft |
| Location | 01-hum-255-r5.13 |
| NBI structure number | 04 0257 |
| Condition class | Good |
| Traffic status (item 41) | Open, no restriction |
| Last inspection (item 90) | September 2023 |
| Inspection interval (item 91) | 48 months |
| Scour (item 113) | 5: foundations stable, scour within the limits of the footing or piles |
Source: FHWA National Bridge Inventory, 2025 data as reported by the California department of transportation.
Busiest bridges in Humboldt County
| Bridge | Built | Vehicles/day | Condition |
|---|---|---|---|
| U.S. Highway 101 over Orchard Ave | 1963 | 25,700 | Good |
| US Highway 101 SB over Palmer Blvd | 1958 | 22,000 | Fair |
| US Highway 101 NB over Palmer Blvd | 1958 | 22,000 | Fair |
| U.S. Highway 101 over US 101 On&off Ramp Ncrra | 1962 | 21,700 | Fair |
| U.S. Highway 101 over Salmon Creek | 1957 | 21,500 | Good |
| U.S. Highway 101 over Finch Creek Road | 1958 | 21,500 | Good |
All bridges nearby: Humboldt County, CA bridge conditions · every bridge in Humboldt County · California bridges.
Background: how bridge ratings work · what "poor" and "structurally deficient" actually mean · how often bridges are inspected.