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Grade separation is the process of aligning a junction (traffic) of two or more transport axes at different heights (grade (slope)s) 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 an
interchange (road), as opposed to an
intersection (road), which is not grade-separated. If there is no grade separation at all, the junction is called "at-grade intersection".
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 List of motorways in the United Kingdom; it is all but impossible on a Interstate Highway System, 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:
Railways
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 Road traffic accident.
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 Intersection (road)) and requires significant engineering effort compared to provision of an at-grade intersection.
See also
.
Grade separation is the process of aligning a
junction (traffic) of two or more transport axes at different heights (grade (slope)s) 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 an
interchange (road), as opposed to an
intersection (road), which is not grade-separated. If there is no grade separation at all, the junction is called "at-grade intersection".
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 List of motorways in the United Kingdom; it is all but impossible on a
Interstate Highway System, 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:
These junctions connect
three or more roads:
- Various incarnations of Spaghetti Junction
These junctions
terminate one road into another:
Railways
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 Road traffic accident.
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
Intersection (road)) and requires significant engineering effort compared to provision of an at-grade intersection.
See also
