An underpass is a road or path that goes underneath another road, railway, or highway. Instead of crossing over the top of busy traffic, you travel beneath it through a tunnel-like passage.

You've probably driven through an underpass without even thinking about it: when your car dips down to go under a bridge while the highway continues above you, that's an underpass. Cities build underpasses to keep traffic flowing smoothly in different directions without cars having to stop for each other. They're especially useful near train tracks, where an underpass lets cars pass underneath while trains rumble overhead.

Pedestrians and cyclists use underpasses too. A walking underpass might let you cross under a dangerous highway safely, emerging on the other side without ever having to dodge traffic. These often have walls made of tile or concrete, with lights to keep them visible and safe.

The opposite of an underpass is an overpass, where the road you're on goes up and over another road instead of dipping below it. Both solve the same problem (how do we cross busy roads safely?) but in opposite directions.