Grade separation
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Grade separation is the process of aligning a junction of two or more transport axes at different heights (grades) so 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 all alike; instead, it can consist of a mixture of roads, railways, and canals. The construction of bridges, tunnels, or a combination of both can be built at a junction to achieve the needed grade separation. In North American terminology, a grade-separated junction is called an interchange, as opposed to an intersection, which is not grade-separated. If there is no grade separation at all, the junction is called "at-grade".
RoadsThe 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) or slip roads (United Kingdom, Ireland) 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. 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 junction designer has placed 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 ring road of Coventry (United Kingdom) is a particularly notorious example, as are parts of the southern M25 motorway (the ring road around London). Weaving can be alleviated by using collector/distributor roads to separate entering and exiting traffic. TypesThese junctions connect two roads:
These junctions connect two roads, but only one is fully grade-separated: These junctions connect three or more roads:
These junctions terminate one road into another: 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 and one which is not a level junction. In 1897, the London and South Western Railway (LSWR) made use of a flying junction at Worting Junction 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 later made extensive use of flying junctions on other parts of its busy former LSWR main line. Today in Britain—although technically a combination of many junctions—the tightly grouped nest of flying junctions[1] to the north of Clapham Junction railway station allow handles of in excess of 4,000 trains per day (about one train every 15 seconds). In the French TGV system, all high-speed junctions are grade-separated. The most interesting of these being the three fully grade-separated high-speed triangles on the LGV system, which 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, United States was completed in 1910. The most frequent use was later found on the former Pennsylvania Railroad main lines. The lines are included as part of the Northeast Corridor and Keystone Corridor now owned by Amtrak. The most complex of these junctions, near Philadelphia Zoo, handles railway traffic for Amtrak, SEPTA, New Jersey Transit, Norfolk Southern, CSX Transportation and Conrail. Advantages and disadvantagesRoads 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
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