Underpasses and overpasses are grade-separated crossings where one transportation route passes under (underpass) or over (overpass) another without intersecting at the same level. Grade separations eliminate conflicts between crossing traffic streams, improving safety and traffic flow. They're used where roads cross railways, where highways intersect, and where major roads cross each other or pedestrian/cycling routes. Interchange ramps use combinations of overpasses and underpasses. Building grade separations requires substantial investment compared to at-grade intersections but eliminates delays and collision risks at crossings. Bridge structures for overpasses require ongoing inspection and maintenance. Height and weight restrictions may apply to underpasses. Grade separation decisions balance costs against traffic volumes, safety concerns, and development patterns.