Editing Grade separation

{{Short description|Type of road junction}}
[[File: Chicago Circle Interchange 2018.jpg|thumb|right|An example of the potential complexity of grade separation, seen in the [[Jane Byrne Interchange]] in [[Chicago]]]]
[[File:Overpasses in Spain.JPG|thumb|Seven various [[overpass]]es for grade separation in [[Spain]] near [[Barcelona]]]]
[[File:201607 Xiaoshan Separation Zone from the Ground.jpg|thumb|Rail-rail grade separation in [[Xiaoshan]], China]]
[[File:Praha-Strašnice, lávka přes trať.jpg|thumb|right|The concept of grade separation includes all transport modes, such as a simple pedestrian bridge over rail tracks.]]

In [[civil engineering]] (more specifically [[highway engineering]]), '''grade separation''' is a method of aligning a [[junction (traffic)|junction]] of two or more surface transport axes at different heights ([[grade (slope)|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 [[road]]s, [[footpath]]s, [[Rail transport|railway]]s, [[canal]]s, or airport [[runway]]s. [[Bridge]]s (or [[overpass]]es, also called flyovers), [[tunnel]]s (or [[underpass]]es), 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''<ref>[http://www.amlegal.com/nxt/gateway.dll/California/eureka/titleviitrafficcode/chapter71trafficrules?f=templates$fn=altmain-nf.htm$3.0#JD_71.85 City of Eureka Municipal Code 71.85] {{Webarchive|url=https://web.archive.org/web/20120212024853/http://www.amlegal.com/nxt/gateway.dll/California/eureka/titleviitrafficcode/chapter71trafficrules?f=templates$fn=altmain-nf.htm$3.0#JD_71.85 |date=2012-02-12 }} (California, US)</ref><ref>{{cite web |url=http://www.ebooksread.com/authors-eng/institute-of-traffic-engineers/traffic-engineering-handbook-hci/page-46-traffic-engineering-handbook-hci.shtml |title=Read the ebook Traffic engineering handbook by Institute of Traffic Engineers |author=Henry K. Evans |year=1950 |work=ENGINEERING HANDBOOK, Second Edition 1950 |publisher=Institute of Traffic Engineers |location=New Haven, Connecticut |access-date=2010-10-09 |archive-date=2018-10-13 |archive-url=https://web.archive.org/web/20181013063246/http://www.ebooksread.com/authors-eng/institute-of-traffic-engineers/traffic-engineering-handbook-hci/page-46-traffic-engineering-handbook-hci.shtml |url-status=dead }}</ref> or as an ''[[interchange (road)|interchange]]'' – in contrast with an ''[[intersection (road)|intersection]]'', ''[[at-grade intersection|at-grade]]'', a ''[[Level junction|diamond crossing]]'' or a ''[[level 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 why [[speed limit]]s are typically higher for grade-separated roads. In addition, reducing the complexity of traffic movements reduces the risk of [[Traffic accident|accidents]].

=== 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 have [[Elevator|lifts]], 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 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]], [[Australia]], [[New Zealand]]) or ''[[slip road]]s'' ([[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 [[freeway]]s, [[highway]]s, [[motorway]]s, or [[dual carriageway]]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 [[List of motorways in the United Kingdom|British motorway]]; it is all but impossible on a [[Interstate Highway System|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''.
[[File:Leipzig-Halle Airport Condor.jpg|thumb|right| A plane on a [[taxiway]] over the [[Autobahn]] at [[Leipzig-Halle Airport]] - a type of grade separation.]]

===Types===
==== Fully separated ====
These junctions connect two freeways:
[[Image:M23-M25 Intersection - geograph.org.uk - 15455.jpg|thumb|4 level stack interchange between the [[M25 motorway|M25]] (below) and [[M23 motorway|M23]] (above) in the [[UK]].]]
* [[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 of [[spaghetti 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]]
* [[Roundabout]] 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

===Weaving===
[[File:Weaving traffic.gif|thumb|right|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, ''weaving'' is 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, the [[cloverleaf interchange]]), or in urban areas with many close-spaced junctions. The [[Coventry ring road|ring road]] of [[Coventry]], [[England]], is a notorious example, as are parts of the southern [[M25 motorway|M25, the London orbital motorway]], the M6/M5 junction north-west of [[Birmingham]], and the A4/M5 junction west of [[Bristol]]. Weaving can often cause side-on collisions on very fast roads with top speeds of up to {{cvt|200|km/h}}, 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 using [[collector/distributor road]]s or braided ramps<ref>{{cite web|title=Braided Ramp|author=Texas Department of Transportation|author-link=Texas Department of Transportation|work=TxDOT Visual Dictionary|url=https://www.txdot.gov/driver/txdot-visual-dictionary/braided-ramp.html|access-date=2020-05-10|archive-date=2020-03-06|archive-url=https://web.archive.org/web/20200306142506/https://www.txdot.gov/driver/txdot-visual-dictionary/braided-ramp.html|url-status=dead}}</ref> to separate entering and exiting traffic.

==== Types ====
The weaving area can be categorized in to three categories based on number of lane changes required for each traffic flow:<ref>{{cite journal |last1=Golob |first1=Thomas F. |last2=Recker |first2=Wilfred W. |last3=Alvarez |first3=Veronica M. |title=Safety aspects of freeway weaving sections |journal=Transportation Research Part A: Policy and Practice |date=1 January 2004 |volume=38 |issue=1 |pages=35–51 |doi=10.1016/j.tra.2003.08.001 |bibcode=2004TRPA...38...35G |access-date=30 March 2025 |url=https://www.sciencedirect.com/science/article/abs/pii/S0965856403000855}}</ref><ref name="Roess2004">{{cite book |last1=Roess |first1=Roger P. |last2=Prassas |first2=Elena S. |last3=McShane |first3=William R. |last4=McShane |first4=William R. |title=Traffic engineering |date=2004 |publisher=Pearson/Prentice Hall |location=Upper Saddle River, N.J |isbn=0-13-142471-8 |pages=341–343 |edition=3rd |chapter=Chapter 13: Weaving, merging, and diverging movements}}</ref>
* Type A configuration: any weaving traffic must change lane at least once.
* Type B configuration: one of the two weaving movement can be completed without a lane change; the other movement requires at most one lane change.
* Type C configuration: one weaving movement has a "through" line without lane change necessary; the other flow must make at least two lane changes.

==== Length ====
In the United States, the length of a weaving area is measured as {{cvt|2|ft}} away from the left edge of the ramp lane edge to a point at the diverge gore area where these two points are separated by {{cvt|12|ft}}. Roess et al. speculates that this measuring practices dates back to a 1963 database assembled by [[Bureau of Public Roads]], when the weaving ramps involved cloverleaf interchanges. In their early designs, the departure angle of the off-ramps is greater than the merge angle of on-ramps.<ref name="Roess2004" />

==Railways==
=== With roads and footpaths ===
{{see also|level crossing}}
In railway construction, grade separation also means the avoidance of [[level crossings]] by making any roads or footpaths crossing the line either pass under or over the railway on [[bridge]]s. 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 for [[high speed railway]] lines and level crossings are increasingly less common due to the increase of both road and rail traffic.<ref>[https://www.networkrail.co.uk/communities/safety-in-the-community/level-crossing-safety/reducing-risk-at-level-crossings/ Reducing risk at level crossings] - [[Network Rail]]. Accessed 27 May 2024</ref> Efforts to remove level crossings are done in the UK by [[Network Rail]] and in [[Melbourne]] as part of the [[Level Crossing Removal Project]].

The [[Great Central Main Line|London Extension]] of the [[Great Central Railway]], built between 1896 and 1899, was the first fully grade-separated railway of this type in the UK. This also applies to [[light rail]] and even to [[streetcar|street cars]].

=== Flying junction ===
{{main|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]].

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 (Great Britain)|Southern Railway]] 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<ref>[http://www.openstreetmap.org/#map=15/51.4760/-0.1440 OpenStreetMap] {{Webarchive|url=https://web.archive.org/web/20110223195827/http://www.openstreetmap.org/#map=15/51.4760/-0.1440 |date=2011-02-23 }}</ref> 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).

Virtually all major railway lines no longer cross (forming an [[Double junction#Diamond|'X' shape]]) at flat level (although many diverge - i.e. 'Y' shape).

=== High-speed railways ({{convert|200|kph|mph|disp=or|abbr=on}}+) ===
On almost all [[high-speed railway]] lines, the faster speed requires grade separation. Therefore, many high speed lines are elevated, especially in [[Taiwan High Speed Rail|Taiwan]] and [[Shinkansen|Japan]], where population density alongside high speed lines is higher than in France, Italy or Germany.

In the United States, a flying junction on the [[New York, Chicago and St. Louis Railroad|Nickel Plate Road]] through [[Cleveland]], [[Ohio]], [[United States]] was completed in 1913.{{Citation needed|reason=The article talks about a grade separation project 1909-1913 but does not mention the flying junction|date=May 2016}} 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]].

In what is known as [[Russian gauge|"area 1520"]], which includes the [[former Soviet Union]] and other regions using the same gauge, the most complicated grade-separation railpoint is found at [[Liubotyn]] in [[Ukraine]].

==Footbridges and subways==
[[Footbridges]] and [[Subway (underpass)|subways]] (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 to [[Central Park]] in New York City in the 1860s, subways are far more common today in Europe, especially in countries such as the [[Cycling in the Netherlands|Netherlands]] and [[Cycling in Denmark|Denmark]] where cycling is strongly encouraged. Long [[underpass]]es may be called [[tunnel]]s.
<gallery class=center>
Image:Bp bridge.JPG| The [[Frank Gehry]]-designed [[BP bridge]] in [[Chicago]] was built for pedestrians.
Image:Centralpark 20040520 121402 1.1504.jpg| Nineteenth-century pedestrian underpass in [[Central Park]]
</gallery>

==References==
{{Reflist}}

{{Road junction types}}

[[Category:Junction types]]