An underpass is a road, path, or railway that passes underneath another road, railway, or structure. Underpasses (along with overpasses) create grade separations that allow different transportation routes to cross without intersecting. Railway underpasses allow roads to cross under tracks without at-grade crossings, improving safety and eliminating train-caused traffic delays. Road underpasses at highway interchanges provide continuous flow without traffic signals. Pedestrian underpasses allow safe crossing under busy roads. Underpass construction is expensive, requiring excavation and drainage systems. Underpass height restrictions limit vehicle sizes—striking low bridges causes significant damage. Many underpasses in older areas have height, width, or flooding constraints that modern design would avoid.