An elevated highway is a controlled-access highway that is raised above grade for its entire length. Elevation is usually constructed as viaducts, typically a long pier bridge. Technically, the entire highway is a single bridge. 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 Alternatives Alternatives to elevated highways are: At grade uncontrolled highways, but these often have safety or traffic flow issues At grade controlled access highways, which may have the issues listed earlier in this article At grade controlled access highways with grade-separated crossings, which may mitigate some or all of the issues listed earlier in the article, at less expense than a fully elevated highway Tunnels, which have similar advantages to elevated highways, but are significantly more expensive to build. Other advantages to tunnels are that they do not occupy as much valuable real estate (since they can be built on at the surface), cause less noise pollution, may cause less long-term environmental damage, and protect travellers from surface weather. Below-grade open cuts, which are typically less costly to maintain, but often (though not always) more expensive to build History Railway predecessors Early engineering for elevated highways owes much to early elevated railway design, which preceded them. Elevated highways were first used to: create free traffic flow prevent accidents in busy cities provide some of the first regional connections between nearby cities, in early arterial traffic plans In the late 19th century and early 20th century, railways and streetcars had frequent accidents where they traversed through population centers. These lead