An overpass is a specific type of bridge, one built to carry traffic over another road or railway. A bridge is the broader category: any structure that spans an obstacle, whether that’s a river, a valley, a canyon, or a highway. Every overpass is a bridge, but not every bridge is an overpass.
The confusion is understandable because the two terms overlap. In everyday conversation, people use them interchangeably. But in engineering and transportation planning, the distinction matters.
What Makes Something a Bridge
A bridge is any structure built for vehicles or people to cross over a road, railway, or body of water. That’s the full scope of the term. The Golden Gate Bridge spans the San Francisco Bay. A small wooden footbridge crosses a creek in a park. A concrete slab carries a two-lane road over a railroad track. All three are bridges, despite looking nothing alike.
Bridges vary enormously in design. They can be suspension bridges, arch bridges, beam bridges, or cable-stayed bridges, and the choice depends on what they’re spanning and how far. A suspension bridge works for long distances over water. A simple beam bridge handles a short crossing over a stream. Some bridges use a single span, others use dozens. The defining feature isn’t the design or size. It’s the function: connecting two points separated by something you can’t just drive or walk across at ground level.
What Makes Something an Overpass
The Massachusetts Department of Transportation defines an overpass as “a bridge that carries a road, railroad, pedestrian or bike facility over another transportation facility.” That last part is the key distinction. An overpass specifically crosses over another route of travel, not a natural obstacle like a river or valley.
Overpasses exist to eliminate intersections. Instead of forcing two busy roads to meet at a stoplight, an overpass lifts one road above the other so traffic flows continuously in both directions. They’re built to manage traffic density, particularly on highways and freeways where stopping at a crossroad would create dangerous bottlenecks. Every highway interchange you’ve driven through uses overpasses to keep merging and exiting traffic separated from through traffic.
The Same Structure, Two Names
Here’s where it gets interesting: whether you call a structure an overpass or an underpass depends entirely on your perspective. California’s Department of Transportation names structures based on the relationship between the roads. If your road goes over the highway, you’re on an overpass (or “overcrossing” in Caltrans terminology). If your road goes under, you’re traveling through an underpass. The physical structure is the same. The name changes based on which road you’re on.
This means a single structure can be an overpass to one driver and an underpass to another, at the exact same moment. The concrete doesn’t care what you call it.
Viaducts: The Long-Distance Cousin
A viaduct is another term that falls under the bridge umbrella. Viaducts carry roads or railways over long distances across valleys, urban areas, or uneven terrain. They’re distinct because of their uniform, repetitive design: a series of evenly spaced columns supporting consistent spans, sometimes stretching for miles. Urban metro rail systems frequently run on viaducts because they maintain a steady elevation above city streets without requiring the city to tear down buildings or reroute existing roads.
The space beneath a viaduct is often put to use for parking, local roads, or commercial space. That efficiency makes them especially practical in dense cities where land is expensive and scarce.
Regional Terms for the Same Thing
If you’ve traveled internationally, you may have heard different words for what Americans call an overpass. In the UK, India, Australia, and much of Africa, the standard term is “flyover.” A British driver might say “take the flyover to avoid the traffic lights,” while an American driver in the same situation would say “take the overpass to avoid the intersection.” The structure itself is identical. The vocabulary is a product of whether your country’s English traces back to British or American conventions.
In Canada and the Philippines, “overpass” is the dominant term, following American English patterns. If you encounter “flyover” in an article or conversation, you can treat it as a direct synonym for overpass.
A Simple Way to Remember
“Bridge” is the family name. It covers every structure that carries people or vehicles over an obstacle. “Overpass” is a specific member of that family, reserved for bridges where the obstacle below is another road, railway, or transportation route. A bridge over a river is just a bridge. A bridge over a highway is an overpass. And if you’re on the highway looking up at it, that same structure is an underpass.