Grade separation
From Freepedia
Grade separation is the process of aligning a junction of two or more transport axes on different levels (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.
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 British motorway.
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 of grade separated junctions
Where at least one road across the junction is grade separated and at least one is not
Where one road terminates on another at a free-flow junction
Where two roads interchange with one another in a free-flow junction
- Stacked interchange (two-level, three-level, or four-level stack, depending on how many levels cross at the central point)
- Cloverleaf interchange
Where more than two roads meet and interchange at a free-flow junction
- Various incarnations of Spaghetti Junction
Railways
Attempts have been made to increace the capacity of railways by making tracks cross in a grade-separated manner, as opposed to the traditional use of points to change tracks. 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.
Personal rapid transit
Personal rapid transit systems also exhibit grade separation where the transit guideway has been elevated to cross road other transport axes throughout its network.
Lack of grade separation
A junction which is not grade separated is at grade or level. See At-grade intersection for more details.
Advantages and dis-advantages
Advantages
- High capacity
- Higher speeds are possible
- Reduced possibility for accidents
Disadvantages
- Higher building and maintainance costs
- Greater space-requirements
