Underpass

n. route or path under a main road (for pedestrians, cars, etc.)

A tunnel is an underground passage. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end.A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water — for consumption, for  hydroelectric purposes or as sewers — while others carry other services such as  telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels or the tunnels connecting the Gaza Strip to Egypt.
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Noun
1. an underground tunnel or passage enabling pedestrians to cross a road or railway
(synonym) subway
(hypernym) tunnel

The grade separation where part or all of one feature instance is directly above part or all of another feature instance. (DOI3)

Synonyms and related words:
access, aisle, alley, ambulatory, aperture, arcade, artery, avenue, channel, cloister, colonnade, communication, conduit, connection, corridor, covered way, defile, exit, ferry, ford, gallery, inlet, interchange, intersection, junction, lane, opening, outlet, overpass, pass, passage, passageway, portico, railroad tunnel, traject, trajet, tunnel