An underpass is a road, path, or tunnel that goes under another road, railway, or obstacle. It lets traffic or pedestrians cross without meeting the traffic above.
From "under" plus "pass," describing a way of passing below something. The term arose with modern road and rail systems.
Underpass and overpass are a neat pair: same crossing, different height. City planners use these words to design traffic in 3D, not just on a flat map.
Complete word intelligence in one call. Free tier — 50 lookups/day.