Why did the truss bridge break up with the beam bridge? It needed more support.
Why did the truss bridge break up with the beam bridge? It needed more support.
Enclosed conduits under roads that carry water or provide passage, often overlooked but incredibly numerous.
Total in US
148,526
Poor Condition
2.4%
Avg Sufficiency
66.3
Average Age
49 yrs
Culverts are the unsung heroes of the American transportation system. Technically classified as bridges when their span exceeds 20 feet (the NBI threshold), culverts are enclosed structures that allow water, wildlife, or other passages beneath a roadway. They are by far the most numerous "bridge" type in many states, though most people would never think of them as bridges at all.
Culverts come in many shapes: circular pipes, box structures, arch shapes, and elliptical cross-sections. They can be made from concrete, corrugated steel, aluminum, plastic, or even stone. Box culverts (rectangular concrete structures) are the most common type that meets the 20-foot bridge threshold, and they handle everything from seasonal stream crossings to major drainage channels.
Despite their humble appearance, culverts play a critical role in flood management, ecosystem connectivity, and road safety. A failed or undersized culvert can wash out a road, cause upstream flooding, or block fish migration. State and federal transportation agencies spend billions of dollars annually on culvert inspection, maintenance, and replacement.
A culvert works by providing a rigid or semi-rigid passage through the soil beneath a road embankment. The structural behavior depends on the culvert type. Box culverts act as rigid frames: the top slab spans between the two walls and resists the weight of the soil and traffic above through bending, while the walls transfer forces to the bottom slab and foundation. Pipe culverts (circular or elliptical) work differently. Flexible pipe culverts (corrugated metal or plastic) rely on the surrounding compacted soil to provide support, with the pipe and soil working together as a system. Rigid pipe culverts (concrete) carry loads primarily through their own structural strength. The hydraulic design is equally important: the culvert must be sized to pass the design flood flow without causing excessive upstream ponding or dangerous outlet velocities.
Some culverts have been specifically designed as wildlife crossings, complete with natural earth floors, vegetation, and even small fences to guide animals toward the passage. In Banff National Park, Canada, wildlife crossing structures have reduced animal-vehicle collisions by over 80%.
Widespread across all US states (tens of thousands)
I-95 drainage culverts throughout the East Coast
Texas box culverts (Texas has more culverts than any state)
Banff wildlife crossing culverts (Canada)
Iconic bridges with a deck hung from cables draped over tall towers, capable of spanning enormous distances.
Modern bridges where cables run directly from towers to the deck, creating a dramatic fan or harp pattern.
One of the oldest bridge forms, using a curved structure to transfer loads outward to abutments at each end.
Bridges built from interconnected triangular elements, combining strength and material efficiency.
| 6,443 |
| 4.3% |
| 6 | Minnesota | 6,184 | 4.2% |
| 7 | Georgia | 5,675 | 3.8% |
| 8 | Missouri | 5,435 | 3.7% |
| 9 | North Carolina | 5,390 | 3.6% |
| 10 | Illinois | 4,981 | 3.4% |