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A causeway is an elevated road on elevated ground, usually across a broad body of water or wetland. A roadway that is carried instead on a system of arches or a bridge is a viaduct. In the U.S. a brief stretch of viaduct is called an overpass. The word causeway is also sometimes used to describe a long, low bridge.
It may be constructed as a cofferdam: two parallel steel sheet pile or concrete retaining walls, anchored to each other with steel cables or rods. This construction may also serve as a dyke that keeps two bodies of water apart, such as bodies with a different water level on each side, or with salt water on one side and fresh water on the other. This may also be the primary purpose of a structure, the road being a side benefit.
Examples of causeways include those that connect Singapore and Venice to the mainland. In the Netherlands there are a number of prominent dykes which double as causeways, including the Afsluitdijk, Brouwersdam, and Markerwaarddijk. In Louisiana, two very long bridges, called the Lake Pontchartrain Causeway, stretch across Lake Pontchartrain for almost 24 miles, making them the world's longest bridges (if total length is considered instead of span length). The oldest engineered road yet discovered is the Sweet Track in England, dating from the 3800s BC.
Causeways are also common in Florida, where low bridges may connect several manmade islands, often with a much higher bridge (or part of a single bridge) in the middle so that taller boats may pass underneath safely. Causeways are most often used to connect the barrier islands with the mainland.
Causeways affect currents and may therefore be involved in beach erosion or changed deposition patterns. Causeways are often a problem with an approaching hurricane or strong tropical storm, because the high winds and waves make them dangerous. Along with traffic jams, this is a major reason for the early evacuation of island residents during a weather emergency.
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