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Grade Separated Junction

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Grade Separated Junction
Junction 8a, M40.jpg
An example of a grade separated junction on the M40
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For detailed information about this topic, see grade separation at

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 turns 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.

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.

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.

Earliest Examples

The majority of the examples of grade separation have come about during the motorway era, however there are several claims on the earliest purpose-built junction, including:

For other examples of early grade-separation see: Category:Early Grade Separated Junctions

Single Carriageway GSJs

In the UK, grade separated junctions are strongly associated with dual carriageways, but examples do exist on single carriageway roads. Most do not have a traditional acceleration lane or taper for safety reasons, and instead use a right-angled 'give way' line.

The start of the A418 at Wheatley would be one example of a single carriageway acceleration lane that has now been painted out. Northern Ireland's Carn Roundabout is an example of an underpass that operates as an S2, as part of an unfinished new town layout. The A38(M)'s midway junction is a dubious example as the road is a single carriageway but not in the traditional sense.

Less questionable examples which function as a grade separated junction include:

  • The junction of the A3 and B2177 near Cosham is almost a cloverleaf, although the underpass is D2 for the duration of the junction only.
  • A181/B1280 at Wingate.
  • A35/A358 works with a link road and right turns.
  • A303/A350 works with a link road and right turns.
  • A45/A361 trumpet near Daventry.
  • A4810/B4245
  • A9/A86 at Kingussie
  • Roads running parallel to, or on the former alignment of, major roads often benefit from grade separation, such as the B7076 (former A74) amd the A168 (parallel to A1(M)).

Grade Separated Junction
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