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Amedian strip,central reservation,roadway median, ortraffic median is the reserved area that separates opposinglanes oftraffic on divided roadways such asdivided 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 bepaved, but commonly it is adapted to other functions; for example, it may accommodate decorativelandscaping,trees, amedian barrier, orrailway,rapid transit,light rail, orstreetcar lines.
There is no international English standard for the term.Median,median strip, andmedian divider island are common in North American andAntipodeanEnglish. Variants inNorth American English include regional terms such asneutral ground inNew Orleans usage orboulevard inVancouver,British Columbia.
InBritish Englishthe 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 examplemiddelmannetjie, which originally referred to the hump between wheel ruts on a dust road).[1][2]Neutral section andcentral nature strip[citation needed] are coinages inAustralian 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 betweenright-hand traffic and left-hand traffic can cause further confusion.
Some medians function secondarily as green areas andgreen belts to beautifyroadways.Jurisdictions can: plantlawn grasses with regularmowing;hydroseed or scatterwildflowerseeds togerminate, bloom, and re-seed themselves annually; or create extensive landscape plantings oftrees,shrubs,herbaceous perennials andornamental grasses. Where space is at a premium, densehedges of shrubs filter theheadlights 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 alight rail orrapid transit line; for example, theRed andBlue Lines of theChicago 'L' partially run in the medians of theDan Ryan,Eisenhower, andKennedy Expressways.[3]
In contrast to the median of a major road, those in urban areas often take the form of centraltraffic islands that rise above the roadway. These are frequently found on urbanarterial 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 asCalifornia, highway medians are sometimes no more than a demarcated section of the paved roadway, indicated by a space between two sets ofdouble 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 StatesInterstate Highways break only for emergency service lanes, with no such restrictions on lower classification roads. On Britishmotorways, the median is never broken (except on thetidal flow ofAston Expressway), but there are no such restrictions on otherdual carriageways.
The central reservation in theUnited 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 theM5 motorway as it climbs up the side of theGordano Valley south ofBristol. InBirmingham 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 theM6 between Shap and Tebay, which allows a local road to run between them, and on theM62 where the highest section through thePennines famously splits wide enough to containa farm.[6] The other major exception is theA38(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 orreversible lane).
With effect from January 2005 and based primarily on safety grounds, the UK'sHighways Agency's policy is that all new motorway schemes are to use high containmentconcrete 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.
InNorth 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 theTrans-Canada Highway nearErnfold,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 insuburban areas and cities. In urban areas, concrete barriers (such asJersey barriers) andguard rails (orguide rails) are used.
InDedham, Massachusetts, theNorfolk County Correctional Center (a state prison) is located entirely within a wide median ofMassachusetts Route 128. This 502-bed facility was opened in 1993 asinfill 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 onInterstate 75 nearCincinnati,Ohio; nearly the entire village ofArlington Heights, as well as the downtown district ofLockland, are both located between the two directions of I-75.
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 ofcontinuous flow intersections, or fordiverging diamond interchanges.
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 inPhiladelphia.
An August 1993 study by the USFederal Highway Administration quantified thecorrelation between median width and the reduction of bothhead-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]
Central reservations may also be used for reservedbus lanes, as in Istanbul'sMetrobus, Los Angeles'sJ Line and Bogotá'sTransMilenio. Center-lane running andisland platforms installed in the medium reduce conflicts with stopped and parked cars as well as pedestrians near the curb, thus speeding service.
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 aright-of-way, because of the expense and difficulty of clearing a route through dense urban neighborhoods. A reserved right-of-way is contrasted withstreet running, in which rail cars and automobiles occupy the same lanes of traffic.
Train lines that run in the median of highways include:
City planners also commonly use median strips to accommodate urbantram tracks along a centralsegregated track, and examples of this layout are found across Europe. Some of the earliest practices of incorporating central tramways intoroad designs were pioneered inLiverpool byJohn Alexander Brodie, and later emulated in Manchester, such as alongPrincess Parkway orKingsway.[8]
