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Cloverleaf

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Cloverleaf
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A Cloverleaf Junction is a large and complex Grade Separated Junction of which very few have been built in the British Isles. In fact, only two now survive as genuine Cloverleaf Junctions, Redditch Cloverleaf in Redditch, Worcestershire and Cousland Interchange in Livingston in West Lothian. A third, Houston Interchange also in Livingston, was modified a few years ago by removing the sliproads from two quadrants.

The Cloverleaf takes its name from the layout of the sliproads, which give the appearance of a four leaf clover held within a squarish frame of outer sliproads. These allow two dual carriageway roads to meet at a fully free-flowing grade separated junction with only one bridge. However, the down side is that there is conflict in the centre of the junction between joining and leaving traffic (although this does mean that you can make a full tour of the four 'leaves' and end up back where you started!).

Common in some countries, the Cloverleaf never gained favour in Britain, and it is perhaps telling that both Redditch and Livingston are New Towns.

There are two further junctions in England which loosely qualify as Cloverleafs, although the layout on a map does not immediately suggest such a design. The first is Crownhill in Plymouth, which has been stretched out along the A386 to minimise weaving, and also had a roundabout added. The second is Coombe Lane Junction on the A3 in south London, which is missing a single sliproad, and has had a C/D Lane added on the northbound A3 to remove weaving.

In addition, Old Inns Junction, now junction 6 on the M80, then a junction on the A80, was for a time in the 1960s and 70s an incomplete cloverleaf, with only one slip road missing (the sharp left turn at the NE corner - a petrol station was in the way). This junction is in Cumbernauld, another new town.

In Germany, where these junctions are particularly common, they are used predominantly on the motorway and expressway network and usually include collector-distributor roads to prevent the slow traffic joining the motorway from interfering with the traffic flow on the main line, as well as to prevent weaving. This increases the cost of the junction by requiring additional and longer bridges to accommodate the extra lanes. An example is on the A2 motorway at Dortmund North East, although there are countless others. Particularly heavily trafficked cloverleafs have also been modified to include directional replacements for one or more of the inner loops - this reduces or removes the requirement for traffic to cross other directional flows and therefore increases overall capacity. An example of this is the A1/A2 Kamener Kreuz.

A final feature of the junction is poor provision for pedestrians and NMUs - intended for motorway-motorway connections, the junction is generally unsuitable for vulnerable road users without separate provision being made owing to the concentration required at merge points for motorists to ensure they change lanes without colliding with one another. On other junction designs, such as the Three Level Stacked Roundabout, where this is signalised, pedestrian phases can be incorporated with little impact to the traffic flow on the junction.

Partially Unrolled Cloverleafs

A Partially Unrolled Cloverleaf Junction is a hybrid between a genuine Cloverleaf design and a Four Level Stack Junction. This results in two opposite 'leaf' loops being removed and replaced with a pair of slips which connect with the left turn slips and generally cross together on one of the corners now missing a loop. This results in each carriageway on the main routes having either two off and one on or one off and two on slips, and so removing the weaving feature of normal Cloverleaf junctions.

Parclos

Parclo is a Sabre abbreviation for a Junction which uses some elements of a cloverleaf design, and is also called a Half Cloverleaf. Often these are between dual carriageways and single carriageway routes, so only half the sliproads are required, with right turn lanes on the minor road to connect with the sliproads. Many of these junctions have had roundabouts added, to form a 'folded Dumbbell interchange.




Cloverleaf
Junction Types
Cloverleaf • Crossroads • Diamond • Directional T • Diverging Diamond • Dumbbell • Fiveways • Four Level Stack • Grade separated • Gyratory • Hamburger Junction • LILO • Longabout • Magic Gyratory • Magic Roundabout • Parclo • Roundabout • Roundabout interchange • Single Point Urban Interchange • Single sliproad junction • T Junction • Trumpet Junction • Whirlpool


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