A tunnel is relatively long and narrow; the length is often much greater than twice the diameter, although similar shorter excavations can be constructed, such as cross passages between tunnels.
The definition of what constitutes a tunnel can vary widely from source to source. For example, the definition of a road tunnel in the United Kingdom is defined as „a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more.“[1] In the United States, the NFPA definition of a tunnel is „An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft).“[2]
In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a subway, while an underground railway system is differently named in different cities, the „Underground“ or the „Tube“ in London, the „Subway“ in Glasgow, and the „Metro“ in Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a bridge or, if passing under a canal, an aqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an underbridge. A longer underpass containing a road, canal or railway is normally called a „tunnel“, whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a „tunnel“, whatever mode of transport it is for.
In the US, the term „subway“ means an underground rapid transit system, and the term pedestrian underpass is used for a passage beneath a barrier. Rail station platforms may be connected by pedestrian tunnels or footbridges.