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Median strip

From Wikipedia, the free encyclopedia
(Redirected from Traffic median)
"Central reservation" redirects here. For other uses, see Central reservation (disambiguation).
See also: Road verge
Typical left-hand motorway road layout in Ireland and South Africa
Divided median strip on a boulevard in Huizhou, China

A median strip, central reservation, roadway median, or traffic median is the reserved area that separates opposing lanes of traffic on divided roadways such as divided highways, dual carriageways, freeways, and motorways. The term also applies to divided roadways other than highways, including some major streets in urban or suburban areas. The reserved area may simply be paved, but commonly it is adapted to other functions; for example, it may accommodate decorative landscaping, trees, a median barrier, or railway, rapid transit, light rail, or streetcar lines.

Regional terminology

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There is no international English standard for the term. Median, median strip, and median divider island are common in North American and Antipodean English. Variants in North American English include regional terms such as neutral ground in New Orleans usage or boulevard in Vancouver, British Columbia.

In British English the central reservation or central median is the preferred usage; it also occurs widely in formal documents in some non-British regions such as South Africa, where there are other informal regional words (for example middelmannetjie, which originally referred to the hump between wheel ruts on a dust road).[1][2] Neutral section and central nature strip[citation needed] are coinages in Australian English.

Additionally, different terminology is used to identify traffic lanes in a multi-lane roadway. North American usage calls the leftmost lanes located closest to the roadway centerline the "inner" lanes, while British usage calls these lanes the "outer" lanes. Thus, it is less confusing to call these central lanes the "passing", "fast", or "overtaking" lanes in international contexts, instead of using the ambiguous inner/outer distinction. Regional differences between right-hand traffic and left-hand traffic can cause further confusion.

Physical attributes

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A grass median strip on Interstate 80 in Pennsylvania, United States

Some medians function secondarily as green areas and green belts to beautify roadways. Jurisdictions can: plant lawn grasses with regular mowing; hydroseed or scatter wildflower seeds to germinate, bloom, and re-seed themselves annually; or create extensive landscape plantings of trees, shrubs, herbaceous perennials and ornamental grasses. Where space is at a premium, dense hedges of shrubs filter the headlights of oncoming traffic and provide a resilient barrier. In other areas, the median may be occupied by a right-of-way for a public transportation system such as a light rail or rapid transit line; for example, the Red and Blue Lines of the Chicago 'L' partially run in the medians of the Dan Ryan, Eisenhower, and Kennedy Expressways.[3]

An arterial road median with decorative cobblestones in Redwood City, California

In contrast to the median of a major road, those in urban areas often take the form of central traffic islands that rise above the roadway. These are frequently found on urban arterial roads. In their simplest form, these are just raised concrete curbs, but can also be landscaped with grass or trees or decorated with bricks or stones. Such medians are also sometimes found on more minor or residential streets, where they serve primarily as a traffic-calming or landscaping element rather than a safety enhancement to restrict turns and separate opposite directions of high-volume traffic flow.

In some areas such as California, highway medians are sometimes no more than a demarcated section of the paved roadway, indicated by a space between two sets of double yellow lines. Such a double-double yellow line or painted median is legally similar to an island median: vehicles are not permitted to cross it, unlike a single set of double yellow lines which may in some cases permit turns across the line.[4] This arrangement has been used to reduce costs, including narrower medians than are feasible with a planted strip, but research indicates that such narrow medians may have minimal safety benefit compared to no median at all.[5]

The medians of United States Interstate Highways break only for emergency service lanes, with no such restrictions on lower classification roads. On British motorways, the median is never broken (except on the tidal flow of Aston Expressway), but there are no such restrictions on other dual carriageways.

Width

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The wide median strip in Main Street, Bairnsdale, Victoria, Australia has been made into a garden feature of the town, wide enough to accommodate features such as flowerbeds and street furniture like benches and postboxes.

The central reservation in the United Kingdom and other densely populated European countries (where it is known by their local names) is usually no wider than a single lane of traffic. In some cases, however, it is extended. For instance, if the road is running through hilly terrain, the carriageways may have to be built on different levels of the slope. An example of this is on the M5 motorway as it climbs up the side of the Gordano Valley south of Bristol. In Birmingham and many other cities, suburban dual carriageways may have trees or cycle lanes in the middle as a wide central reservation.

Two examples on the UK road network where the carriageways are several hundred yards/metres apart, are on a section of the M6 between Shap and Tebay, which allows a local road to run between them, and on the M62 where the highest section through the Pennines famously splits wide enough to contain a farm.[6] The other major exception is the A38(M) Aston Expressway, which is a single carriageway of seven lanes, where the median lane moves to account for traffic flow (a system known as tidal flow or reversible lane).

With effect from January 2005 and based primarily on safety grounds, the UK's Highways Agency's policy is that all new motorway schemes are to use high containment concrete step barriers in the median (central reservation). All existing motorways will introduce concrete barriers as part of ongoing upgrades and through replacement as and when the current systems have reached the end of their useful life. This change of policy applies only to barriers in the median of high speed roads and not to verge side barriers. Other routes will continue to use steel barriers.

In North America, and some other countries with large sparsely populated areas, opposing lanes of traffic may be separated by several hundred meters of fields or forests outside of heavily populated areas (an extreme example being the Trans-Canada Highway near Ernfold, Saskatchewan, Canada, where eastbound and westbound lanes go as far as 5 kilometres (3.1 mi) apart from each other), but converge to a lane's width of separation in suburban areas and cities. In urban areas, concrete barriers (such as Jersey barriers) and guard rails (or guide rails) are used.

In Dedham, Massachusetts, the Norfolk County Correctional Center (a state prison) is located entirely within a wide median of Massachusetts Route 128. This 502-bed facility was opened in 1993 as infill construction in the previously unused real estate that had been isolated by the divided highway in the early 1950s. An extreme example of a wide median can be found on Interstate 75 near Cincinnati, Ohio; nearly the entire village of Arlington Heights, as well as the downtown district of Lockland, are both located between the two directions of I-75.

A 50-metre-wide median strip in Inagi, Tokyo.[citation needed]

Inverted median

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Some freeways in North America include "inverted" medians, which separate roadways running in the opposite direction from the standard for the country they are located in. Roads are so designed for a number of reasons, including to save space, for the creation of continuous flow intersections, or for diverging diamond interchanges.

  • The north–south Golden State Freeway (I-5) in the Tehachapi Mountains of California between Los Angeles and the San Joaquin Valley. For several miles, the direction of traffic on both sides of the median is reversed — northbound traffic is on the western roadway and southbound traffic on the eastern roadway.
  • I-85 in central North Carolina between Lexington and Thomasville features an inverted median, constructed so that an in-median, right-exit rest area can feature a historic bridge.
  • I-77 in Charlotte, North Carolina, has a reverse median at its intersection over I-85 for on- and off-ramps without taking too much land from neighboring properties and without building many more bridges.
  • I-84 used to have an inverted median through Waterbury, Connecticut, east of the 2-level "mixmaster" interchange with Route 8. This section of freeway was reconstructed in the 1970s, which eliminated the inverted median.

Inverted medians are also used in rare cases on local streets that historically had unusual traffic patterns, such as Bainbridge Street between 3rd Street and 5th Street in Philadelphia.

Safety impact

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Concrete median barrier on the N11 road near Dublin, Ireland

An August 1993 study by the US Federal Highway Administration quantified the correlation between median width and the reduction of both head-on accidents and severe injuries. The study found that medians without barriers should be constructed more than 30 feet (9.1 m) wide in order to have any effect on safety, and that safety benefits of wider medians continue to increase to a width of 60 to 80 feet (18.3 to 24.4 m).[5]

A consequence of this finding is that decreasing the size of a median to 20 feet (6.1 m) from 30 feet (9.1 m) to add lanes to a highway may result in a less safe highway. Statistics regarding medians with barriers were not calculated in this study.[5]

Bus rapid transit

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Central reservations may also be used for reserved bus lanes, as in Istanbul's Metrobus, Los Angeles's J Line and Bogotá's TransMilenio. Center-lane running and island platforms installed in the medium reduce conflicts with stopped and parked cars as well as pedestrians near the curb, thus speeding service.

Railway use

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In some cases, the median strip of the highway may contain a train line, usually around major urban centers. This is often done to share a right-of-way, because of the expense and difficulty of clearing a route through dense urban neighborhoods. A reserved right-of-way is contrasted with street running, in which rail cars and automobiles occupy the same lanes of traffic.

Train lines that run in the median of highways include:

Asia

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Europe

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North America

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Canada

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United States

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California

Oceania

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South America

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City planners also commonly use median strips to accommodate urban tram tracks along a central segregated track, and examples of this layout are found across Europe. Some of the earliest practices of incorporating central tramways into road designs were pioneered in Liverpool by John Alexander Brodie, and later emulated in Manchester, such as along Princess Parkway or Kingsway.[8]

See also

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References

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  1. ^ Branford, Jean & Branford, William. A Dictionary of South African English. Oxford University Press 1992 ISBN 978-0195705959
  2. ^ Bosman, D. B.; Van der Merwe, I. W.; Hiemstra, L. W. (1984). Tweetalige Woordeboek Afrikaans-Engels. Tafelberg-uitgewers. ISBN 978-0-624-00533-9.
  3. ^ Buck, Thomas (September 28, 1969). "Ryan Rail Service Starts Today". Chicago Daily Tribune. p. 22.
  4. ^ "Crossing double yellow lines", Spokesman-Review, April 28, 2014.
  5. ^ a b c Federal Highway Administration (August 1993). "The Association Of Median Width And Highway Accident Rate". Retrieved 2008-01-09.
  6. ^ Google Maps
  7. ^ Ortofoto (Map). Lantmäteriet. Retrieved 17 August 2018.
  8. ^ Crosby, Alan, ed. (1998). Leading the way : a history of Lancashire's roads. Preston: Lancashire County Books. p. 212. ISBN 9781871236330.
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