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An elevated highway is a
controlled-access highway A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms i ...
that is raised above grade for its entire length. Elevation is usually constructed as
viaduct A viaduct is a specific type of bridge that consists of a series of arches, piers or columns supporting a long elevated railway or road. Typically a viaduct connects two points of roughly equal elevation, allowing direct overpass across a wide v ...
s, 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 In mathematics and transportation engineering, traffic flow is the study of interactions between travellers (including pedestrians, cyclists, drivers, and their vehicles) and infrastructure (including highways, signage, and traffic control devi ...
, 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 Noise pollution, also known as environmental noise or sound pollution, is the propagation of noise with ranging impacts on the activity of human or animal life, most of them are harmful to a degree. The source of outdoor noise worldwide is main ...
, 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 to the first " death avenues", such as 11th Avenue in New York City. Aside from safety, carts and pedestrians crossing trains' paths slowed service.Staff.&nbs
"State may regulate tracks in 11th Ave.; Court Denies Right of City to Disturb New York Central in Use of the Street."
''
The New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid ...
'', May 20, 1911. Accessed August 7, 2009. "...the way had been opened through the decision for settling the so-called 'Death Avenue' problem".
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.


Dawn of the automobile

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 Julius Miller (January 12, 1880 – February 3, 1955) was a judge and politician in New York City. He was a public figue for fuur decades, seving as Manhattan Borough President from 1922 to 1930, and as a New York State Supreme Court judge from ...
, 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 World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, several extensions were built from 1947 to 1951, under the leadership of urban planner
Robert Moses Robert Moses (December 18, 1888 – July 29, 1981) was an American urban planner and public official who worked in the New York metropolitan area during the early to mid 20th century. Despite never being elected to any office, Moses is regarded ...
, primarily connecting it to his other projects, such as the
Henry Hudson Parkway The Henry Hudson Parkway is a parkway in New York City. The southern terminus is in Manhattan at 72nd Street, where the parkway continues south as the West Side Highway. It is often erroneously referred to as the West Side Highway throughout it ...
and Brooklyn-Battery Tunnel. The Miller Highway influenced many other subsequent projects, such as Boston's
Central Artery The Central Artery (officially the John F. Fitzgerald Expressway) is a section of freeway in downtown Boston, Massachusetts; it is designated as Interstate 93, US 1 and Route 3. The original Artery, constructed in the 1950s, was named after ...
and the
Pulaski Skyway The Pulaski Skyway is a four-lane bridge-causeway in the Gateway Region, northeastern part of the U.S. state of New Jersey, carrying an freeway, expressway designated U.S. Route 1/9 (US 1/9) for most of its length. The structure has a ...
, and Moses' own
Gowanus Parkway Interstate 278 (I-278) is an auxiliary Interstate Highway in New Jersey and New York in the United States. The road runs from US Route 1/9 (US 1/9) in Linden, New Jersey, northeast to the Bruckner Interchange in the New York ...
. At the start the 20th century,
New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...
and
New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
state officials realized that car traffic on ferries was increasing beyond the ability of the then-current ferry system. Planning for the
Holland Tunnel The Holland Tunnel is a vehicular tunnel under the Hudson River that connects the New York City neighborhood of Hudson Square in Lower Manhattan to the east with Jersey City in New Jersey to the west. The tunnel is operated by the Port Author ...
started in 1919, and it was constructed from 1922 to 1927. As construction started, New Jersey began planning traffic flows between the tunnel and nearby cities. The
legislature A legislature is an assembly with the authority to make law Law is a set of rules that are created and are enforceable by social or governmental institutions to regulate behavior,Robertson, ''Crimes against humanity'', 90. with its p ...
passed a bill to extend existing highway Route 1 east through
Newark Newark most commonly refers to: * Newark, New Jersey, city in the United States * Newark Liberty International Airport, New Jersey; a major air hub in the New York metropolitan area Newark may also refer to: Places Canada * Niagara-on-the ...
and
Jersey City Jersey City is the second-most populous city in the U.S. state of New Jersey, after Newark.Pulaski Skyway The Pulaski Skyway is a four-lane bridge-causeway in the Gateway Region, northeastern part of the U.S. state of New Jersey, carrying an freeway, expressway designated U.S. Route 1/9 (US 1/9) for most of its length. The structure has a ...
for the eastern portions of the new route (until close to the entrance of the tunnel). It opened in 1933. Like the Miller Highway, it included left lane exits and entrances, narrow lanes, and local surface lanes underneath the highway. The Miller Highway, through immature design and resulting problems, became a case study for highway engineering improvements. Engineering of paving, exit orientation, turn radius, drainage, curb height, ramp length, speed optimization, shoulders, maintenance procedures, noise abatement, and minimizing the "highway wall" effect that could divide communities, were all improved in the 1940s through 1970s, partially by examining the deficiencies of this early elevated highway.


The Interstate system

In the mid-1930s, US Federal Highway legislation allocated budget for surveying and planning of roads, including "superhighways", across the nation, and ordered the chief of the Bureau of Public Roads to report findings and recommendations. The report, submitted in 1939, included a master plan, which recommended that urban highways be "depressed or elevated". In 1944,
President Franklin D. Roosevelt Franklin Delano Roosevelt (; ; January 30, 1882April 12, 1945), often referred to by his initials FDR, was an American politician and attorney who served as the 32nd president of the United States from 1933 until his death in 1945. As the ...
submitted a follow-on report, ''Interregional Highways'', which contained illustrations of the depressed and elevated designs. The elevated illustration, reminiscent of the Miller Highway and some of its descendants, featured partial left lane ramps, a highway running across the width a populated boulevard, almost building-to-building, a local traffic lanes underneath the highway. The report also includes a picture of the then-recently constructed Gowanus Parkway, and noted how it was thought to have been an appropriate placement that had a minimal effect on the community. (This was later disputed, and the Gowanus is also a case study for how elevated highways divide neighborhoods and contribute to urban blight.) The phrase ''Interregional Highways'' gave way to ''Interstate Highways'' in a Congressional bill in 1944, and included $125 million for urban highways. The lead agency, the Federal Public Roads Administration (PRA) worked with state engineer associations to develop planning and design criteria. The PRA's leaders, especially Thomas H. MacDonald and
Herbert S. Fairbank Herbert S. Fairbank, referred to by colleagues as Jack, worked for the U.S. Bureau of Public Roads under Thomas Harris MacDonald and contributed to major reports on highway infrastructure in the U.S. He served as chairman of the Highway Transport ...
, were especially concerned about urban highways. Design standards were issued, with some opposition, which were significant improvements over existing designs. For example, minimum lane width of 12 feet and with a median of at least 4 feet (later standards would increase median sizing), minimal overpass heights were set at 14 feet, recommendations to acquire right of way sufficient for proper ramps of 3° great or less, right-side exit only and only to arterial connections. The Interstate standards have set the pace in the United States for optimal highway design, including those of elevated highways.


Today

Elevated expressways are now common around the world, particularly in the central urban areas where traffic volumes and urban densities are high such as cities in the Americas, East and Southeast Asia. Entire networks of elevated expressways exist in the central areas of cities such as
Metro Manila Metropolitan Manila (often shortened as Metro Manila; fil, Kalakhang Maynila), officially the National Capital Region (NCR; fil, link=no, Pambansang Punong Rehiyon), is the capital region, seat of government and one of three List of metrop ...
,
Guangzhou Guangzhou (, ; ; or ; ), also known as Canton () and alternatively romanized as Kwongchow or Kwangchow, is the capital and largest city of Guangdong province in southern China. Located on the Pearl River about north-northwest of Hong Kon ...
,
Bangkok Bangkok, officially known in Thai language, Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand. The city occupies in the Chao Phraya River delta in central Thailand and has an estima ...
,
Osaka is a designated city in the Kansai region of Honshu in Japan. It is the capital of and most populous city in Osaka Prefecture, and the third most populous city in Japan, following Special wards of Tokyo and Yokohama. With a population of 2. ...
,
Shanghai Shanghai (; , , Standard Mandarin pronunciation: ) is one of the four direct-administered municipalities of the People's Republic of China (PRC). The city is located on the southern estuary of the Yangtze River, with the Huangpu River flow ...
,
Tokyo Tokyo (; ja, 東京, , ), officially the Tokyo Metropolis ( ja, 東京都, label=none, ), is the capital and largest city of Japan. Formerly known as Edo, its metropolitan area () is the most populous in the world, with an estimated 37.468 ...
, and
Wuhan Wuhan (, ; ; ) is the capital of Hubei, Hubei Province in the China, People's Republic of China. It is the largest city in Hubei and the most populous city in Central China, with a population of over eleven million, the List of cities in China ...
.


See also

*
Elevated railway An elevated railway or elevated train (also known as an el train for short) is a rapid transit railway with the tracks above street level on a viaduct or other elevated structure (usually constructed from steel, cast iron, concrete, or bricks ...


References

{{Road junction types Types of roads