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'Grade separation' is the process of aligning a junction of two or more transport axes at different heights (grades) such that they do not disrupt the flow of traffic on one another when they cross. This is achieved by building bridges over or tunnels under the crossing site, allowing roads, railways and canals to pass another without interrupting the flow of traffic.
In North American terminology, a junction which is grade-separated is an ''interchange'', as opposed to an ''intersection'' which is not grade-separated; a lack of grade separation is described as "at-grade".

Contents

Roads

Weaving

Types

Railways

Advantages and disadvantages

See also

Roads

The term is most widely applied to describe a road junction where 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, Australia, etc.) or ''slip roads'' (United Kingdom, Ireland, etc.) 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, highways, motorways or dual carriageways 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 very uncommon to find an at-grade junction on a British motorway; it is all but impossible on a U.S. Interstate Highway, 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''.

Weaving

''Weaving'' 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 junction design places the on-slip to the road before the off-slip at a junction (for example, the cloverleaf interchange), or in urban areas with lots of close-spaced junctions (the Coventry ring road is a particularly notorious example, as are parts of the southern M25 motorway).
Weaving can be alleviated by using collector/distributor roads to separate entering and exiting traffic.

Types

These junctions connect 'two roads':

★ Stack interchange (two-level, three-level, or four-level stack, depending on how many levels cross at the central point)

★ Cloverleaf interchange

★ 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.
These junctions connect 'two roads', but 'only one is fully grade-separated':

★ Diamond interchange

★ Parclo interchange

★ Single Point Urban Interchange

★ Roundabout Interchange
These junctions connect 'three or more roads':

★ Various incarnations of Spaghetti Junction
These junctions 'terminate one road into another':

★ Trumpet interchange

★ Directional-T interchange

Railways

Main articles: Flying junction

Attempts 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 and one which is not a level junction.
The London and South Western Railway (LSWR) made use of a flying junction at Worting Junction south of Basingstoke in 1897 to allow the Salisbury and Southampton routes to join/converge without conflicting movements; this became known as Battledown Flyover. The Southern Railway later made extensive use of flying junctions on other parts of its busy former LSWR main line.
One of the earliest examples of this type in the USA was on the Nickel Plate Road through Cleveland, Ohio, United States, completed in 1910; one of the most recent and complex is the Sandgate Flyover at Sandgate, New South Wales, Australia. The most frequent use of flying junctions is on the former Pennsylvania Railroad main lines that are now part of the Northeast Corridor and Keystone Corridor owned by Amtrak. The most complex of the junctions, near the Philadelphia Zoo, handles train traffic for Amtrak, SEPTA, New Jersey Transit, Norfolk Southern, CSX Transportation and Conrail. In the French TGV high-speed train system, all junctions are grade-separated.

Advantages and disadvantages

Roads with grade separation generally allow traffic to move freely, with less interruptions, and at higher overall speeds; hence why speed limits are typically higher for grade-separated roads. In addition, less conflict between traffic movements reduces the capacity for accidents.
Grade-separated junctions are however large, and costly. Their height can be obtrusive, and 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 NIMBY groups for these reasons. The United States suffered an extended period of anti-grade separation protests known as the freeway and expressway revolts.
Grade-separation are very expensive, time-consuming, potentially uses up to three times more space, depending on the required space (Compared with at grade intersections) and requires significant engineering effort compared to provision of an at-grade intersection.

See also