underbridge

Noun

 * 1)  A bridge that allows traffic to pass under a road, river, railway etc.
 * 2) The underneath of a bridge.
 * 1) The underneath of a bridge.
 * 1) The underneath of a bridge.

Usage notes
Where a bridge takes one form of transport over another it is both an underbridge and an overbridge, depending on the reference level. For example, where a road passes above a railway, the bridge is an overbridge from the point of view of the railway, and an underbridge from the point of view of the road.