Elevated highway

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.

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:

Alternatives to elevated highways are:

Early engineering for elevated highways owes much to early elevated railway design, which preceded them. Elevated highways were first used to:

In the late 19th century and early 20th century, railways and streetcars had frequent accidents where they traversed through population centers. These lead to the first "death avenues," such as 11th Avenue in New York City. Aside from safety, carts and pedestrians crossing trains' paths slowed service.

In addition, it became difficult to lay down rail lines, as the construction process was disruptive to normal traffic flow. The existing street grid also made it difficult to lay some railroad lines, as the trains required a wide turn radius.

This led to the first elevated railways in the late 19th century. The elevated rails, being grade-separated, prevented almost all pedestrian/vehicle accidents, and could allow track bends above existing structures. Their construction could still be disruptive, but was usually less so, as pier construction to support their elevated structures did not necessarily close an entire roadway or long stretches of roadway for an extended period. However, conversion from at grade railways to elevated (or below ground) did not always take place, and many lines continued to be at grade in urban areas well into the 20th century.

Concurrently, the increase of automobile and truck traffic early in the 20th century exacerbated many of the safety and free flow issues the railways already presented - and in fact, created additional hazards with railways. The increase in traffic also meant that for the first time, there was a need to develop new and improved roads between cities.

By the 1920s, truck traffic in warehouse and dock areas was high enough that there was frequent congestion and frequent accidents. In 1924, New York City began looking for ways to relieve the problems of the combination of trucks, cars, trains, and pedestrians on 11th Avenue, which had been known as Death Avenue even before the advent of the car and truck. The mayor, the Manhattan borough president, the police commissioner, the Port Authority, the New York Central Railroad (owner of the West Side Line whose tracks were on 11th Avenue), and others worked on various plans to take the railroad and passenger cars off the street, eliminating the major conflicts that led to injury, death, property damage, traffic jams, and delays in service. The Miller Highway, named after its chief proponent, Borough President Julius Miller, was constructed in sections, primarily from 1929 through 1937, and became the world's first elevated, controlled access highway. After an interruption for World War II, several extensions were built from 1947 to 1951, under the leadership of urban planner Robert Moses, primarily connecting it to his other projects, such as the Henry Hudson Parkway and Brooklyn-Battery Tunnel.

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