Grade Separated Junction
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A Grade Separated Junction (GSJ) is a junction where the conflicting traffic flows are kept apart, usually by means of a bridge or tunnel. They are one of the most effective ways of solving traffic congestion, as they do not directly require any traffic to slow down and stop at any time.
Whilst some such junctions are fully freeflowing for all movements, many are merely grade separated for the more important route, with the lesser route(s) still having to negotiate right tuns or a roundabout.
The opposite of a GSJ, where conflicting traffic flows meet in the same place, is known as an at-grade junction.
Note that an Interchange is a catch-all term used to cover the vast array of unique designed junctions where Grade Separation is included.
Freeflowing GSJ Types
Main Article: Cloverleaf
This is a large, junction type typically used where two main routes cross at right angles. It is called a cloverleaf junction because slip roads curl round through 270 degrees , merging above or below where they demerge in each corner, giving the appearance of a 4-leaf clover. The opposite movement is provided with a straight slip passing around the outside of each 'leaf'. It is not favoured in the UK as it causes conflict between on and off slips, and only a handful have been built, with some being incomplete.
Main Article: Trumpet Junction
A trumpet interchange is a grade separated version of a T Junction, where one road terminates on another. slips on the nearside of the through route are easily provided, with a bridge then crossing over to the far side, and the two slips curving round to left or right to meet the main road. This causes one slip to pass through 270 degrees.
Main Article: Whirlpool
A whirlpool interchange is a design which uses free flowing sliproads to join two dual carriageways, but with a design such to minimise land use, the number of bridges required, and traffic conflicts. an example is the Theydon Bois Interchange
Main Article: Four Level Stack
A four level stack interchange is a design which uses free flowing sliproads to join two dual carriageways, but with a design such to remove traffic conflicts and tight corners/sliproads. An example is the Colnbrook Interchange.
Main Article: Directional T junction
Directional T Junctions are a grade separated variation of the humble T junction design, where one of the routes terminates at the junction, its carriageways splitting into sliproads to meet the through route. Some of the more important grade separated Ts are actually non-Directional T Junctions...
Other GSJ Types
Main Article: Diamond interchange
A Diamond interchange is perhaps the simplest of GSJ types, where the sliproads from the main route meet the lesser route at simple give-way junctions. This forces around half of the traffic to turn right, either onto or off of the sliproads. These days, many are controlled by Traffic Lights, whilst a few have been converted to Dumbbell junctions (below).
Main Article: Dumbbell interchange
A Dumbbell interchange is where instead of using a single roundabout, requiring two bridges, two roundabouts are used on either side of a single bridge. Whilst further slowing the flow of traffic on the lesser route, it is cheaper to build, taking up less land and with only one bridge to build.
Main Article: Roundabout interchange
This is a Roundabout where at least one of the main routes passes through unobstructed, with a pair of bridges either carrying the main road over the roundabout, or carrying the two sides of the roundabout over the main route. Some Motorways Interchanges use a Roundabout Interchange, with two main routes passing through, and a roundabout providing the connections.