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An elevated highway is a controlled-access highway that is raised above grade for its entire length. Elevation is usually constructed as viaduct == Reason to construct == Elevated highways are more expensive to build than at-grade highways, and are usually only used where there is some combination of the following on the desired route: * difficulty controlling access at grade, for example where it would be very disruptive or expensive to eliminate existing crossings at grade * at grade construction would not allow for optimal traffic flow, for example due to hilly terrain or existing crossings * budget or time to eliminate impeding structures is high, due to acquisition costs, demolition costs, or environmental factors; for example: * * right of way through an urban area, where private property would have to be purchased or condemned, and might have to be litigated * * hills that are costly to level or carve a path through * * protected wetlands where foundations and paving may cause unacceptable environmental damage, or mitigation would be long and expensive * a safety issue at grade, for example, where there are many pedestrians or wildlife 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Elevated highway」の詳細全文を読む スポンサード リンク
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