Crossing under
An underpass exists when a traffic route, a pipeline or an electrical line is laid under an obstacle.
In many cases it is more expensive than crossing it , but it has the advantage that the clearance profile of the obstacle is not restricted (important for port entrances). Traffic routes that cross busy waterways are often found in the form of underwater tunnels , for example in the Elbe tunnel in Hamburg.
In addition, underpasses often offer city planners the only way to reconcile the requirements of a modern transport network with the preservation of structures that have evolved over time.
Power lines pass under obstacles in the form of underground or submarine cables . They are used when an overhead line crossing is impractical.