Grade separation

Grade separation is a method of aligning a junction of two or more surface 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 (or overpasses or flyovers), tunnels (or underpasses), 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[1][2] or as an interchange – in contrast with an intersection, at-grade, a diamond crossing or a level crossing, which are not grade-separated.

Circle Interchange Chicago
An example of the potential complexity of grade separation, seen in the Circle Interchange in Chicago
Overpasses in Spain
Seven various overpasses for grade separation in Spain near Barcelona
Praha-Strašnice, lávka přes trať
The concept of grade separation includes all transport modes, such as a simple pedestrian bridge over rail tracks.



Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why speed limits are typically higher for grade-separated roads. In addition, less trouble between traffic movements reduces the risk of accidents.


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.

Grade-separation can create accessibility problems for people with disabilities due to the vertical gradient required to pass or to reach rail platforms.

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.



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 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 extremely 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.


Fully separated

These junctions connect two freeways:

M23-M25 Intersection - - 15455
4 level stack interchange between the M25 (below) and M23 (above) in the UK.

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:

Other variants

These junctions connect three or more roads:

These junctions terminate one road into another:


Weaving traffic
An example of weaving, where traffic drives on the left. The blue car entering the grade separated road and the red 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 ring road of Coventry, England, is a notorious example, as are parts of the southern 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 200 kilometres per hour, as well as the problem of blind spots.

Weaving can be alleviated by using collector/distributor roads to separate entering and exiting traffic.


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 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[3] 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 'X' shape) at flat level (although many diverge - i.e. 'Y' shape).

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

On almost all high-speed railway lines, the faster speed requires grade separation. Therefore, many high speed lines are elevated, especially in China and 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 Nickel Plate Road through Cleveland, Ohio, United States was completed in 1913. 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 "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.

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 bridges. This greatly improves safety and is crucial to the safe operation of high-speed lines. The 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.

Footbridges and subways

Footbridges and pedestrian/cyclist subways (called underpasses in North America as well as in the United Kingdom when referring to roads) are often employed to allow pedestrians and cyclists to cross busy streets and highways. 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 Netherlands, and Denmark where cycling is strongly encouraged. Long underpasses may be called tunnels.

Bp bridge

The Frank Gehry-designed BP bridge in Chicago was built for pedestrians.

Centralpark 20040520 121402 1.1504

Nineteenth-century pedestrian underpass in Central Park


  1. ^ City of Eureka Municipal Code 71.85 (California, USA)
  2. ^ Henry K. Evans (1950). "Read the ebook Traffic engineering handbook by Institute of Traffic Engineers". ENGINEERING HANDBOOK, Second Edition 1950. New Haven, Connecticut: Institute of Traffic Engineers. Retrieved 2010-10-09.
  3. ^ OpenStreetMap
Broadway station (Caltrain)

Broadway is a Caltrain station in Burlingame, California. Caltrain only serves the stop on weekends and holidays; weekday service is provided by a bus shuttle to Millbrae.

Canterbury railway station, Melbourne

Canterbury railway station is located on the Lilydale and Belgrave, lines in Victoria, Australia. It serves the eastern Melbourne suburb of Canterbury, and opened on 1 December 1882.In the 1930s, a crossover was provided at the southern end of the station, and a signal box provided to work the interlocked level crossing gates.Work on the grade separation of the Canterbury Road level crossing commenced in early 1966, and was completed on 15 September 1968. The works involved the use of a temporary track to keep trains running, and raising the station six metres. Services on the third track from East Camberwell were extended though the station to Box Hill in December 1971.

Chowk Kumharanwala Level II Flyover

Chowk Kumharanwala Level II Flyover also called Jinnah Chowk Flyover is located in Multan city of Pakistan, at an intersection itself called as Chowk Kumharanwala, Jinnah Chowk or Qadaffi Chowk.

Foundation stone was put by former prime minister Yousaf Raza Gillani and ground breaking of level I flyover was also done by Yousaf Raza Gillani on 26 April 2011.

It was constructed as part of Inner Ring Road Multan project.


DN5 (Romanian: Drumul Naţional 5) is an important national road in Romania which links Bucharest with the southern country border with Bulgaria by the Giurgiu Russe Friendship Bridge.DN5 has been designated as a priority express road, being upgraded between 2006 and 2009. Between Bucharest and Giurgiu, for the most part, it is a non-grade separated dual carriageway with no emergency lane, with entrances and exits from adjacent roads using roundabouts. Some segments (such as the Adunaţii Copăceni bypass) are constructed to full motorway standard (dual carriageway, full grade-separation, wide median separation).It is one of the most transited roads in southern Romania, serving as the main connection between Romania's capital, Bulgaria and the rest of Southeast Europe.

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". A burrowing junction or dive-under occurs where the diverging line passes below the main line.

The alternative to grade separation is a level junction or flat junction, where tracks cross at grade, and conflicting routes must be protected by interlocked signals.

Gardiner railway station

Gardiner railway station is located on the Glen Waverley line, in Victoria, Australia. It serves the eastern Melbourne suburb of Glen Iris, and opened on 24 March 1890.The station buildings were rebuilt in 1975, and have since been demolished, as part of the grade separation project.

In July 2015, a grade separation project, to replace the Burke Road road/tramway crossing, commenced. The station was demolished in September 2015, with a temporary station opening soon after. On 18 January 2016, the new below ground station opened.A signal box was located on Platform 2, at the Down end of the station, to control the road/tramway crossing. It was abolished on 2 January 2016, during the grade separation works.

The station is among sixteen others that are the site of eleven storey high communication towers, aimed to improve identification of trains around the network, and emergency management.

Greater Grand Crossing, Chicago

Greater Grand Crossing is one of the 77 community areas of Chicago, Illinois. It is located on the city's South Side.

Hawksburn railway station

Hawksburn railway station is located on the Pakenham, Cranbourne and Frankston lines, in Victoria, Australia. It opened on 7 May 1879, and serves the south-eastern Melbourne suburb of South Yarra.

The station is named after the Hawksburn Estate in which it was built. The current station buildings were completed in 1914, as part of the duplication and grade separation of the railway between South Yarra and Caulfield.

Hillsdale station (Caltrain)

Hillsdale is one of three Caltrain stations in San Mateo, California. The station is next to the Bay Meadows neighborhood and close to the Hillsdale Shopping Center.

Manitoba Highway 190

Provincial Trunk Highway 190 (PTH 190), also known as CentrePort Canada Way, is a provincial highway in the Canadian province of Manitoba. It connects the Perimeter Highway with Winnipeg Route 25 (Inkster Boulevard) in the City of Winnipeg; part of the highway passes through the neighboring Rural Municipality of Rosser. The highway includes a traffic interchange at the junction with the Perimeter Highway, as well as a grade separation over the CP Rail main line. The highway is intended to service the industrial lands west of James Armstrong Richardson Winnipeg International Airport, which will be a part of the CentrePort Canada cargo hub. It is numbered for its ultimate role in connecting Highway 1 with Winnipeg Route 90.

PTH 190 is one of four three-digit urban expressway routes in the Manitoba highway network.

CentrePort Canada Way was opened to traffic on November 22, 2013. The expressway was officially opened at a ribbon cutting ceremony attended by Prime Minister Stephen Harper, Manitoba Premier Greg Selinger and Diane Gray, CentrePort Canada President and CEO.

Mashhad Urban Railway

Mashhad Urban Railway (Persian: قطار شهری مشهد‎, the literal translation of the name from Persian) is a rapid transit urban rail line in Mashhad, Iran. It is the second rapid transit system in Iran. The project has been known by a number of terms, including "light rail" or "light metro" and "urban rail" or "metro", though the system's full-grade separation from traffic and five-minute headway fully qualify it as a rapid transit or "metro" system. Mashhad Urban Railway operates its line 1 from 6 to 22:00 daily. Construction of the second line which is a metro line is ongoing. Limited operation of the first phase of line 2, with 10.1 kilometres (6.3 mi) and 9 stations, had just begun in Feb 2017. tunnel excavation of the first phase of line 3 was started in 2015.

Construction of line 1 began in 1999, with the inauguration of Line 1 taking place on 24 April 2011.

Newcastle Link Road

Newcastle Link Road is a limited-access road that links Newcastle, New South Wales to the Pacific Motorway and Sydney.

The route was constructed in December 1993 as a part of the Palmers Road to Minmi section of the Pacific Motorway through former coal mining lands, improving connectivity between Newcastle and Sydney and relieving the Pacific Highway through Charlestown, Belmont and Swansea of a large amount of traffic heading to and coming from Sydney and further south.

As part of the construction of the Hunter Expressway, the interchange of the Pacific Motorway and Newcastle Link Road was upgraded with the addition of three new bridges next to the original single bridge, maintaining the previous grade separation conditions and directly connecting into the Hunter Expressway.In preparation of the increased demand on Newcastle Link Road from the construction of the Hunter Expressway, the intersections with Cameron Park Drive / Woodford Street and Lake Road / Thomas Street were upgraded from roundabouts to become traffic light controlled. The route was also signed with the A15 number in early 2014 to reflect the increased status of this route into Newcastle.

Nunawading railway station

Nunawading railway station is located on the Lilydale and Belgrave railway lines in Victoria, Australia, and serves the eastern Melbourne suburb of Nunawading. The station was called Tunstall when it opened on 4 June 1888, but was renamed Nunawading on 1 November 1945.In 1979 the station was one of three used as trial sites for new bike lockers under the Melbourne Bicycle Strategy, either for occasional, monthly or quarterly hire.As part of the grade separation project to replace the Springvale Road level crossing, the original ground-level station closed on 18 December 2009 and was replaced by a new station in a cutting west of Springvale Road, which opened on 11 January 2010.


An overpass (called an overbridge in the United Kingdom and Australia, and a flyover in the United Kingdom as well as some other Commonwealth countries) is a bridge, road, railway or similar structure that crosses over another road or railway. An overpass and underpass together form a grade separation. Stack interchanges are made up of many overpasses.

Panhandle Bridge

The Panhandle Bridge (officially the Monongahela River Bridge) carries two rail lines of the Port Authority "T" line across the Monongahela River in Pittsburgh, Pennsylvania. The name comes from Pennsylvania Railroad subsidiary Pittsburgh, Cincinnati, Chicago and St. Louis Railroad, also known as the Panhandle Route, which operated over the bridge.

The basic structure was built in 1903, and was the third railroad bridge on the site since 1863. It was raised in 1912-14 as part of a grade separation project. The bridge's function was to carry Panhandle Route passenger, mail and express trains from Pennsylvania Station in Pittsburgh, with a tunnel in between the station and the bridge. Pennsy Panhandle freight trains utilized the Ohio Connecting Bridge slightly downstream on the Ohio River, or went the long way around the West Virginia Panhandle via Conway, Pennsylvania.

Rail traffic over the Panhandle Bridge declined as passenger trains were discontinued, and Amtrak became the only regular user of the bridge from 1971 to 1979, when the New York-St. Louis-Kansas City National Limited was discontinued on October 1 of that year. As PRR successor Conrail had no use for the bridge and the restrictive downtown tunnel, it was sold to the Port Authority, who rebuilt the bridge beginning in 1982 as part of the downtown light rail subway project, which removed trolleys from downtown streets and the Smithfield Street Bridge. PAT (as the Port Authority system was known at the time) light rail cars began using the bridge on July 7, 1985.

Pedestrian separation structure

A pedestrian separation structure is any structure that removes pedestrians from a roadway, street or railway track. This creates a road junction where vehicles and pedestrians do not interact.

This can be considered a type of grade separation structure on the road.

These structures can be located either above the roadway or below the roadway. In the U.S., access under the Americans with Disabilities Act requirements means that stairs cannot be the only access to these structures. An elevator must be provided or a ramp built that conforms to the grade requirements under the ADA regulations.

In the broadest sense, building codes that limit the number of driveways that cross sidewalks may be viewed as making the sidewalks a separation structure.

In many areas, wildlife crossings are provided in wilderness areas to allow wildlife to cross roadways without risking accidents. While not specifically built for people, they could be used by people in those areas.

Reservoir railway station

Reservoir railway station is located on the Mernda line in Victoria, Australia. It serves the north-eastern Melbourne suburb of Reservoir, and opened on 8 October 1889 as Preston-Reservoir. It was renamed Reservoir on 1 December 1909.The station was the terminus for suburban services on the Whittlesea line, with the line duplicated in December 1910, and in July 1921, it became the terminus for electric trains. In 1924, an 8 metre long turntable was provided at the station, for the operation of an AEC railmotor between Reservoir and Whittlesea station, running two trips a day. Electric trains were extended to Thomastown in December 1929, and the turntable was abolished in 1940.

Boom barriers replaced hand gates at the High Street level crossing in 1963.On 8 May 1988, the former signal box and interlocked frame were abolished. Also abolished were crossovers, formerly located at the Up and Down ends of the station, and the double line block system between Reservoir and Keon Park, which was replaced with automatic three position signalling. Pedestrian boom barriers were also provided at the pedestrian crossing, located at the Down end of the station, and at the High Street level crossing. Two months earlier, the double line block system between Bell and Reservoir was abolished.The current High Street level crossing was provided in 1991.On 25 June 1996, it was upgraded to a premium station.In January 2016, it was announced by the Level Crossing Removal Authority that the High Street level crossing will be removed by grade separation. Preliminary designs were released in September 2018 showing the grade separation as elevated rail with a new station in the current location. Plans for grade separation have dated as far back as the mid 1970s.

San Bruno station (Caltrain)

San Bruno is a Caltrain station located in San Bruno, California. The station is located just northeast of downtown San Bruno, above the intersection of San Mateo and San Bruno Avenues, adjacent to Artichoke Joe's Casino.

West Side Line

The West Side Line, also called the West Side Freight Line, is a railroad line on the west side of the New York City borough of Manhattan. North of Penn Station, from 34th Street, the line is used by Amtrak passenger service heading north via Albany to Toronto; Montreal; Niagara Falls and Buffalo, New York; Rutland, Vermont; and Chicago. South of Penn Station, a 1.45-mile (2.33 km) elevated section of the line abandoned since 1980 has been transformed into an elevated park called the High Line. The south section of the park from Gansevoort Street to 20th Street opened in 2009 and the second section up to 30th Street opened in 2011, while the final section to 34th Street opened in 2014.

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