Median strip

From Wikipedia, the free encyclopedia
  (Redirected from Central reservation)
Jump to: navigation, search
"Central reservation" redirects here. For other uses, see Central reservation (disambiguation).
Typical motorway road layout (Irish, Israeli and South African road markings)

The median strip is the reserved area that separates opposing lanes of traffic on divided roadways, such as divided highways, dual carriageways, freeways, or motorways. The term also applies to divided roadways other than highways, such as 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 or streetcar lines.

Regionally-preferred terminology[edit]

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 parkway[1] and esplanade, plus regional terms such as neutral ground in New Orleans usage.

In British English central reservation 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.[2][3] Among other coinages, central nature strip occurs in Australian English.

Additionally, different terminology is used to identify traffic lanes in a multi-lane roadway. North American usage calls the 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[edit]

A grass median strip on a highway 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.[4]

A California arterial road median with decorative cobblestones inset

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. Similar to an island median, vehicles are only permitted to cross at designated locations. 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[edit]

The wide median strip in Main St, Bairnsdale, Victoria, Australia has been made into a garden feature of the town.

The median strip in the United Kingdom (where it is known as a central reservation) 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.

Two examples on the UK road network where the carriageways are several hundred yards/meters 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.

A raised concrete median can be seen dividing US Route 52 in Rochester, Minnesota

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 which had been isolated by the divided highway in the early 1950s.

Inverted median[edit]

One median of note is the "inverted" median of 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. A similar example exists in western Montreal, on the Autoroute 20, between the Route 138 and the Turcot Interchange, where the two directions, on opposite sides of a railway, are reversed (and one enters or exits via the left side). Similarly, I-85, in central North Carolina, features an inverted median, constructed so that an in-median, right-exit rest area can feature a historic bridge.

Safety impact[edit]

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]

See also[edit]

References[edit]

  1. ^ Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002. http://unabridged.merriam-webster.com (14 Apr. 2007).
  2. ^ Branford, Jean & Branford, William. A Dictionary of South African English. Oxford University Press 1992 ISBN 978-0195705959
  3. ^ Bosman, D. B.; Van der Merwe, I. W.; Hiemstra, L. W. (1984). Tweetalige Woordeboek Afrikaans-Engels. Tafelberg-uitgewers. ISBN 0-624-00533-X. 
  4. ^ Buck, Thomas (September 28, 1969). "Ryan Rail Service Starts Today". Chicago Daily Tribune. p. 22. 
  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