Grade separation is the method of aligning a junction
A junction, when discussed in the context of transport, is a location where traffic can change between different routes, directions, or sometimes modes, of travel.-Historical significance:...
of two or more transport axes at different heights (grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges, tunnels, or a combination of both can be built at a junction to achieve the needed grade separation.
In North America, a grade-separated junction may be referred to as a grade separation or as an interchange
In the field of road transport, an interchange is a road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream. It differs from a standard intersection, at which...
– in contrast with an intersection
An intersection is a road junction where two or more roads either meet or cross at grade . An intersection may be 3-way - a T junction or fork, 4-way - a crossroads, or 5-way or more...
or an at-grade
An at-grade intersection is a junction at which two or more transport axes cross at the same level .-Traffic management:With areas of high or fast traffic, an at-grade intersection normally requires a traffic control device such as a stop sign, traffic light or railway signal to manage conflicting...
, which are not grade-separated.
AdvantagesRoads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why speed limit
Road speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...
s are typically higher for grade-separated roads. In addition, less conflict between traffic movements reduces the capacity for accidents. Motorways, though having higher average speeds, usually have much lower accident rates per distance travelled than roads which are not grade separated.
DisadvantagesGrade-separated junctions are very space-intensive, complicated, and costly, due to the need for large physical structures such as tunnels, ramps, and bridges. Their height can be obtrusive, and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents. New grade-separated road plans can receive significant opposition from local groups for these reasons.
Rail-over-rail grade separations take up less space than road grade separations; because shoulders are not needed, there are generally fewer branches and side road connections to accommodate (because a partial grade separation will accomplish more improvement than for a road), and because at-grade railway connections often take up significant space on their own. However, they require significant engineering effort, and are very expensive and time-consuming to construct.
Rail-over-road grade separations require very little additional space because no connections need be built, but they require significant engineering effort and are expensive and time-consuming to construct.
OverviewThe term is most widely applied to describe a road junction in which the direct flow of traffic on one or more of the roads is not disrupted. Instead of a direct connection, traffic must use on and off ramps (United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
) or slip roads (United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
) to access the other roads at the junction. The road which carries on through the junction can also be referred to as grade separated.
Typically, large freeways, highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
s, motorways, or dual carriageway
A dual carriageway is a class of highway with two carriageways for traffic travelling in opposite directions separated by a central reservation...
s are chosen to be grade separated, through their entire length or for part of it. Grade separation drastically increases the capacity of a road compared to an identical road with at-grade junctions. For instance, it is extremely uncommon to find an at-grade junction on a British motorway; it is all but impossible on a U.S. Interstate Highway
Interstate Highway System
The Dwight D. Eisenhower National System of Interstate and Defense Highways, , is a network of limited-access roads including freeways, highways, and expressways forming part of the National Highway System of the United States of America...
, though a few do exist.
If traffic can traverse the junction from any direction without being forced to come to a halt, then the junction is described as fully grade separated or free-flowing.
Fully separatedThese junctions connect two roads:
- Stack interchangeStack interchangeA stack interchange is a free-flowing grade separated junction between two roads.In countries where one drives on the right, left turns are handled by semi-directional flyover/under ramps...
(two-level, three-level, or four-level stack, depending on how many levels cross at the central point)
- Cloverleaf interchangeCloverleaf interchangeA cloverleaf interchange is a two-level interchange in which left turns, reverse direction in left-driving regions, are handled by ramp roads...
- Compact grade-separation, whereby the two roads are linked by a compact "connector road", with major-minor priority junctions at each of its ends; usually a variant of the cloverleaf type interchange, but only involving two quadrants rather than four
Partially separatedThese junctions connect two roads, but only one is fully grade-separated, i.e. traffic on one road does not have to stop at yield lines or signals on one road, but may have to do so when switching to the other:
- Diamond interchangeDiamond interchangeA diamond interchange is a common type of road junction, used where a freeway crosses a minor road. The freeway itself is grade-separated from the minor road, one crossing the other over a bridge...
- Parclo interchangeParclo interchangeA partial cloverleaf interchange or parclo is a modification of a cloverleaf interchange. The parclo interchange was developed by the Ontario Ministry of Transportation as a replacement for the cloverleaf on 400-Series Highways, removing the dangerous weaving patterns and allowing for more...
- Single-point urban interchangeSingle-point urban interchangeA single-point urban interchange , also called a single-point interchange or single-point diamond interchange , is a type of highway interchange...
- RoundaboutRoundaboutA roundabout is the name for a road junction in which traffic moves in one direction around a central island. The word dates from the early 20th century. Roundabouts are common in many countries around the world...
Other variantsThese junctions connect three or more roads:
- Various incarnations of Spaghetti JunctionSpaghetti Junction"Spaghetti Junction" is a nickname sometimes given to a complicated or massively intertwined road traffic interchange that resembles a plate of spaghetti. The term is believed to have been coined by a journalist at the Birmingham Evening Mail in the 1970s to refer to the Gravelly Hill Interchange...
These junctions terminate one road into another:
- Trumpet interchange
- Directional-T interchange
WeavingWeaving is a consequence of having too many grade separated junctions on a road in a short distance, where traffic wanting to leave the grade-separated road at the next junction has to fight for road space with traffic which has just entered from the previous one.
This situation is most prevalent either where the junction designer has placed the on-slip to the road before the off-slip at a junction (for example, the cloverleaf interchange
A cloverleaf interchange is a two-level interchange in which left turns, reverse direction in left-driving regions, are handled by ramp roads...
), or in urban areas with lots of close-spaced junctions. The ring road
A ring road, orbital motorway, beltway, circumferential highway, or loop highway is a road that encircles a town or city...
Coventry is a city and metropolitan borough in the county of West Midlands in England. Coventry is the 9th largest city in England and the 11th largest in the United Kingdom. It is also the second largest city in the English Midlands, after Birmingham, with a population of 300,848, although...
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
is a notorious example, as are parts of the southern M25 motorway
The M25 motorway, or London Orbital, is a orbital motorway that almost encircles Greater London, England, in the United Kingdom. The motorway was first mooted early in the 20th century. A few sections, based on the now abandoned London Ringways plan, were constructed in the early 1970s and it ...
(the ring road around London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
) and the M6/M5 junction in Birmingham.
Weaving can be alleviated by using collector/distributor roads to separate entering and exiting traffic.
RailwaysAttempts have been made to increase the capacity of railways by making tracks cross in a grade-separated manner, as opposed to the traditional use of flat crossings to change tracks. A grade-separated rail interchange is known as a flying junction
A flying junction or flyover is a railway junction at which one or more diverging or converging tracks in a multiple-track route cross other tracks on the route by bridge to avoid conflict with other train movements. A more technical term is "grade-separated junction"...
and one which is not a level junction
In U.S. railroad practice, a level junction is a railway junction that has a track configuration in which merging or crossing railroad lines provide track connections with each other that require trains to cross over in front of opposing traffic at grade In U.S. railroad practice, a level junction...
In 1897, the London and South Western Railway
London and South Western Railway
The London and South Western Railway was a railway company in England from 1838 to 1922. Its network extended from London to Plymouth via Salisbury and Exeter, with branches to Ilfracombe and Padstow and via Southampton to Bournemouth and Weymouth. It also had many routes connecting towns in...
(LSWR) made use of a flying junction at Worting Junction
Worting Junction is a railway junction on the former LSWR route south of Basingstoke where the line divides to go towards Salisbury or Southampton.-History:...
south of Basingstoke to allow traffic on the Salisbury and Southampton routes to converge without conflicting movements; this became known as "Battledown Flyover". Also in Britain, the Southern Railway
Southern Railway (Great Britain)
The Southern Railway was a British railway company established in the 1923 Grouping. It linked London with the Channel ports, South West England, South coast resorts and Kent...
later made extensive use of flying junctions on other parts of its busy former LSWR main line.
Today in Britain, the tightly grouped nest of flying junctions to the north of Clapham Junction railway station—although technically a combination of many junctions—handle more than 4,000 trains per day (about one train every 15 seconds).
In the French TGV
The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator....
system, all high-speed junctions are grade-separated. The three fully grade-separated high-speed triangles on the LGV system are capable of being taken at speeds between 160 km/h–320 km/h (100 mph–200 mph).
In the United States, a flying junction on the Nickel Plate Road through Cleveland, Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
, United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
was completed in 1910. The most frequent use was later found on the former Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....
main lines. The lines are included as part of the Northeast Corridor
The Northeast Corridor is a fully electrified railway line owned primarily by Amtrak serving the Northeast megalopolis of the United States from Boston in the north, via New York to Washington, D.C. in the south, with branches serving other cities...
and Keystone Corridor
The Keystone Corridor is a Federal Railroad Administration "designated high speed corridor" with a 349-mile railroad line between Philadelphia, Pennsylvania and Pittsburgh, Pennsylvania with a top speed of...
now owned by Amtrak
The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...
. The most complex of these junctions, near Philadelphia Zoo
The Philadelphia Zoo, located in Philadelphia, Pennsylvania, on the west bank of the Schuylkill River, was the first zoo in the United States. Chartered by the Commonwealth of Pennsylvania on March 21, 1859, its opening was delayed by the American Civil War until July 1, 1874...
, handles railway traffic for Amtrak, SEPTA, New Jersey Transit
New Jersey Transit
The New Jersey Transit Corporation is a statewide public transportation system serving the United States state of New Jersey, and New York, Orange, and Rockland counties in New York State...
, Norfolk Southern, CSX Transportation
CSX Transportation operates a Class I railroad in the United States known as the CSX Railroad. It is the main subsidiary of the CSX Corporation. The company is headquartered in Jacksonville, Florida, and owns approximately 21,000 route miles...
, and Conrail.
In the former Soviet Union (also known as "area 1520"
In railway terminology, Russian gauge refers to railway track with a gauge between 1,520 mm and . In a narrow sense as defined by Russian Railways it refers to gauge....
), the most complicated grade-separation railpoint is found at Liubotyn
Lyubotyn is a city in the Kharkiv Oblast of eastern Ukraine. It is designated as a separate raion .The current estimated population is around 24,173 .-External links:* * * **— Official website of Liubotyn City Council...
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
In railway construction, grade separation also means the avoidance of level crossings by making any roads crossing the line either pass under or over the railway on bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
s. This greatly improves safety and is crucial to the safe operation of high-speed lines. The London Extension
Great Central Main Line
The Great Central Main Line , also known as the London Extension of the Manchester, Sheffield and Lincolnshire Railway , is a former railway line which opened in 1899 linking Sheffield with Marylebone Station in London via Nottingham and Leicester.The GCML was the last main line railway built in...
of the Great Central Railway
Great Central Railway
The Great Central Railway was a railway company in England which came into being when the Manchester, Sheffield and Lincolnshire Railway changed its name in 1897 in anticipation of the opening in 1899 of its London Extension . On 1 January 1923, it was grouped into the London and North Eastern...
, built between 1896 and 1899, was the first fully grade-separated railway of this type in the UK.