bridge.

or overpass.

If driving over it on a straight or level grade, it is an overpass. Technically an overpass is a road that allows one to drive over another road making a level and distinct grade separation.

If driving under it with a change in grade, it is an underpass. An underpass generally changes its grade to sink under another road, by excavating downward beneath the higher road.

If it is a long, level bridge with multiple spans, columns, or arches, it is a viaduct. Think of elevated rail lines in Chicago, or long bridges across wide rivers supported by several spans of piers or arches.