Grade separation
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An example of a four-level stack interchange in the Netherlands. |
Grade separation is the process of aligning a
junction of two or more transport axes at different heights (
grades) such that they do not disrupt the flow of traffic on one another when they cross. This is achieved by building
bridges over or
tunnels under the crossing site, allowing
roads,
railways and
canals to pass another without interrupting the flow of traffic.
A junction which is grade-separated is an
interchange, as opposed to an
intersection which is not grade-separated; a lack of grade separation is described as "
at-grade".
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
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.
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 prelevant 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).
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 interchangeThese 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 JunctionThese junctions
terminate one road into another:
*
Trumpet interchange*
Directional-T interchange:''See main article:
Flying junctionAttempts 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 points to change tracks. A grade-separated rail interchange is known as a
flying junction and one which is not a
level junction.
One of the earliest examples of this type was one 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 are 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.
Personal rapid transit systems also exhibit grade separation where the transit guideway has been elevated to cross road other transport axes throughout its network.
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
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 is expensive, time-consuming and requires significant engineering effort compared to provision of an at-grade intersection.
