Editing Viaduct (section)

==Over land==
[[File:Ouse Valley Viaduct 02.jpg|thumb|left|upright=1.1|The [[Ouse Valley Viaduct]] in Sussex, England]]

The longest viaduct in [[ancient history|antiquity]] may have been the [[Pont Serme]] which crossed wide marshes in southern France.<ref>Colin O’Connor: Roman Bridges, Cambridge University Press 1993, {{ISBN|0-521-39326-4}}, p. 99</ref> At its longest point, it measured 2,679 meters with a width of 22 meters.

Viaducts are commonly used in many cities that are railroad [[transport hub|hub]]s, such as Chicago, Birmingham, London and [[Manchester]]. These viaducts cross the large [[railroad yard]]s that are needed for [[freight train]]s there, and also cross the multi-track railroad lines that are needed for heavy rail traffic. These viaducts provide [[grade separation]] and keep highway and city street traffic from having to be continually interrupted by the train traffic. Likewise, some viaducts carry railroads over large valleys, or they carry railroads over cities with many cross-streets and avenues.
[[File:Viaduct near Slade, Plymouth - Nicholas Matthew Condy - 60 1937 1.jpg|thumb|Viaduct near Slade, Plymouth, painting by [[Nicholas Matthews Condy|Nicholas Matthew Condy]]]]
Many viaducts over land connect points of similar height in a landscape, usually by bridging a river valley or other eroded opening in an otherwise flat area. Often such valleys had roads descending either side (with a small bridge over the river, where necessary) that become inadequate for the traffic load, necessitating a viaduct for "through" traffic.<ref>Brownlee, Christy (March 2005) "Taking the high road: France's new bridge helps a small town dodge traffic—and set a new world record" ''SuperScience'' 16(6): pp.12–15;</ref> Such bridges also lend themselves for use by rail traffic, which requires straighter and flatter routes.<ref>Davidsen, Judith (April 1993)  "A new "lite" rail viaduct formula: Norman Foster designs a rapid-transit viaduct for Rennes, France" ''Architectural Record'' 181(4): p.26;</ref> Some viaducts have more than one deck, such that one deck has vehicular traffic and another deck carries rail traffic.  One example of this is the [[Prince Edward Viaduct]] in Toronto, Canada, that carries motor traffic on the top deck as [[Bloor Street]], and metro as the [[Bloor–Danforth line|Bloor-Danforth]] [[Toronto subway|subway line]] on the lower deck, over the steep [[Don River (Ontario)|Don River]] [[Toronto ravine system|valley]]. Others were built to span settled areas, crossing over roads beneath—the reason for many viaducts in London.