Twin bridges are a set of two bridges running parallel to each other. A pair of twin bridges is often referred to collectively as a twin-span or dual-span bridge. Bridges of this type are often created by building a new bridge parallel to an existing one in order to increase the traffic capacity of the crossing. While most twin-span bridges consist of two identical bridges, this is not always the case. The two bridges that form the Chesapeake Bay Bridge, for example, have several differences, most notably in the number of lanes each carries. The longest twin-span bridge is the Lake Pontchartrain Causeway in Louisiana.
For a bridge owner, twin bridges can improve the maintenance and management of the structures. For motorists, twin bridges can limit the risk that both directions of traffic will be disrupted by an accident.
- Guidance for Good Bridge Design (Google books). Luasanne, Switzerland: Fédération Internationale du Béton. 2000. p. 15. ISBN 2-88394-049-5. Retrieved October 3, 2009.
In order to ensure better management and maintenance of such structures, but also in order to limit the risk that the bridge must be closed in cases of serious highway accidents, most authorities impose a solution of twin bridges which are totally independent of each other. It is clear that in most common cases this is a reasonable and judicious solution.
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