Grade separation

An example of the potential complexity of grade separation, seen in theJane Byrne InterchangeinChicago

Seven variousoverpassesfor grade separation inSpainnearBarcelona

Rail-rail grade separation inXiaoshan,China

The concept of grade separation includes all transport modes, such as a simple pedestrian bridge over rail tracks.

Incivil engineering(more specificallyhighway engineering),grade separationis a method of aligning ajunctionof two or more surface transport axes at different heights (grades) so that they will not disrupt thetraffic flowon 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 ofroads,footpaths,railways,canals,or airportrunways.Bridges(oroverpasses,also called flyovers),tunnels(orunderpasses), 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 agrade separationor as aninterchange– in contrast with anintersection,at-grade,adiamond crossingor alevel crossing,which are not grade-separated.

Effects

Advantages

Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is whyspeed limitsare typically higher for grade-separated roads. In addition, reducing the complexity of traffic movements reduces the risk ofaccidents.

Disadvantages

Grade-separated road junctions are typically 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. For these reasons, proposals for new grade-separated roads can receive significant public opposition.

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.

Grade-separated pedestrian and cycling routes often require modest space since they do not typically intersect with the facility (such as a highway) that they cross. However, grade-separated pedestrian crossings with steps introduce accessibility problems. Some crossings havelifts,but these can be time-consuming to use.

Grade-separated roads that permit for higher speed limits can actually reduce safety due to 'weaving' (see below) as well as a perceived sense of safety.

Roads

Overview

The term is most widely applied to describe aroad junctionin which the direct flow of traffic on one or more of the roads is not disrupted. Instead of a direct connection, traffic must useonandoff ramps(United States,Australia,New Zealand) orslip 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 asgrade separated.

Typically, largefreeways,highways,motorways,ordual carriagewaysare 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 aBritish motorway;it is all but impossible on aU.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 asfully grade separatedorfree-flowing.

A plane on ataxiwayover theAutobahnatLeipzig-Halle Airport- a type of grade separation.

Types

Fully separated

These junctions connect two freeways:

4 level stack interchange between theM25(below) andM23(above) in theUK.

• Stack interchange(two-level, three-level, or four-level stack, depending on how many levels cross at the central point)
• Cloverleaf interchange
• Trumpet interchange
• Cloverstack interchange
• Directional T
• Semi-directional T
• Turbine (whirlpool) interchange
• Windmill interchange
• Various incarnations ofspaghetti junction

Partially separated

These 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 interchange
• Partial cloverleaf interchange
• Single-point urban interchange
• Roundaboutinterchange
• 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

Weaving

An example of weaving, where traffic drives on the left. The blue car entering the grade-separated road, and both the red and blue car exiting must both change lanes in the short distance provided.

On roadways with grade-separated interchanges,weavingis a result of placing an exit ramp a short distance after an entry ramp, causing conflicts between traffic attempting to leave the roadway at the next junction and traffic attempting to enter from the previous junction. 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, thecloverleaf interchange), or in urban areas with many close-spaced junctions. Thering roadofCoventry,England,is a notorious example, as are parts of the southernM25, the London orbital motorway,the M6/M5 junction north-west ofBirmingham,and the A4/M5 junction west ofBristol.Weaving can often cause side-on collisions on very fast roads with top speeds of up to 200 kilometres per hour, as well as the problem of blind spots.

Where junctions have unusual designs weaving can be a problem other than on the main road. An example of this can be found at Junction 7 of the M6, where traffic joining the roundabout from the M6 Eastbound off-slip must weave with the traffic already on the roundabout wishing to use the M6 Westbound on-slip. This is as a result of the slip roads on the west side of the junction connecting to the roundabout on the inside of the eastern arc rather than the outside of the western arc as is normal. The two slip-roads are connected by a single lane on the inside of the roundabout, which traffic wishing to use the Westbound on-slip must join, and traffic from the Eastbound off-slip must leave.

Weaving can be alleviated by usingcollector/distributor roadsor braided rampsto separate entering and exiting traffic.

Railways

With roads and footpaths

See also:level crossing

In railway construction, grade separation also means the avoidance oflevel crossingsby making any roads or footpaths crossing the line either pass under or over the railway onbridges.This greatly improves safety and is crucial to the safe operation of high-speed lines. The construction of new level crossings is generally not permitted, especially forhigh speed railwaylines and level crossings are increasingly less common due to the increase of both road and rail traffic.Efforts to remove level crossings are done in the UK byNetwork Railand inMelbourneas part of theLevel Crossing Removal Project.

TheLondon Extensionof theGreat Central Railway,built between 1896 and 1899, was the first fully grade-separated railway of this type in the UK. This also applies tolight railand even tostreet cars.

Flying junction

Main article: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 aflying junctionand one which is not alevel junction.

In 1897, theLondon and South Western Railway(LSWR) made use of a flying junction atWorting Junctionsouth 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, theSouthern Railwaylater 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 junctionsto the north ofClapham Junction railway station—although technically a combination of many junctions—handle more than 4,000 trains per day (about one train every 15 seconds).

Virtually all major railway lines no longer cross (forming an'X' shape) at flat level (although many diverge - i.e. 'Y' shape).

High-speed railways (200 km/h or 120 mph+)

On almost allhigh-speed railwaylines, the faster speed requires grade separation. Therefore, many high speed lines are elevated, especially inTaiwanandJapan,where population density alongside high speed lines is higher than in France, Italy or Germany.

In the United States, a flying junction on theNickel Plate RoadthroughCleveland,Ohio,United Stateswas completed in 1913.The most frequent use was later found on the formerPennsylvania Railroadmain lines. The lines are included as part of theNortheast CorridorandKeystone Corridornow owned byAmtrak.The most complex of these junctions, nearPhiladelphia Zoo,handles railway traffic for Amtrak,SEPTA,New Jersey Transit,Norfolk Southern,CSX Transportation,andConrail.

In what is known as"area 1520",which includes theformer Soviet Unionand other regions using the same gauge, the most complicated grade-separation railpoint is found atLiubotyninUkraine.

Footbridges and subways

Footbridgesandsubways(called underpasses in North America as well as in the United Kingdom when referring to roads) may be employed to allow pedestrians and cyclists to cross busy or fast streets. They are often used over and under motorways since at grade pedestrian crossings are generally not permitted. Same can be said for railways. Though introduced toCentral Parkin New York City in the 1860s, subways are far more common today in Europe, especially in countries such as theNetherlands,andDenmarkwhere cycling is strongly encouraged. Longunderpassesmay be calledtunnels.

• 
  TheFrank Gehry-designedBP bridgeinChicagowas built for pedestrians.
• 
  Nineteenth-century pedestrian underpass inCentral Park