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Junctions

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A simple at-grade junction
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A junction is the meeting-point ot two or more roads. At its most basic, it may be a T-junction without markings, lighting, or signage; at its most complex, a large multi-lane, multi-level, grade-separated junction.

Junctions may have names and/or numbers: see Junction numbers for more information.

This page is intended to give a brief outline of the different types of Junctions found on the British and Irish road network. Please follow the links below to gain more detail on a specific type.

At Grade or Grade Separated Junctions

The term 'Grade' in terms of junction design refers to the vertical alignment of the various roads which meet at the junction. At Grade means that all of the roads are on the same level, while Grade Separated means that one or more of the routes passes over another. 'Flat' is a synonym for 'at grade'. The vast majority of junctions are therefore At Grade, including T junctions, crossroads, and flat roundabouts. Grade Separated Junctions are most commonly found on Motorways and our busiest dual carriageways, although there are a few to be found on single carriageway roads as well. Grade Separating a junction does not necessarily mean that the junction is completely free flow. The main route will have free flow, and would normally not have a right turn requirement for traffic, but the lesser route passing under or over the main route will often have give way lines and right turns. This makes the junction cheaper to build while still maintaining freeflow on the busier route.

There is also a hybrid type, normally found where two single carriageway routes cross, where the two through routes are uninterrupted by aid of a bridge, while a curved link road provides a connection between them. The A9 in northern Scotland has a number of such junctions, including the Dalmore Junction with the B817 at Alness, which has one connecting road, and the Kingussie Interchange with the A86 which has two connecting roads, removing right turns on the A9 itself.

At Grade Junctions

T Junctions and Forks

Main Article: T Junction
A stop line at a T Junction near Castledawson in Northern Ireland

The simplest junction is where 3 roads meet on the level. These are commonly referred to as T Junctions, or Fork Junctions if there is an acute angle between two of the roads. These junctions are by far and away the most common on our road networks, being widespread in rural and urban areas alike. The busiest of these junctions, particularly in urban areas, have generally had traffic lights installed in order to control traffic flows, particularly where the main traffic flow includes a right turn. While most T Junctions have Give Way lines on the road at right angles, there are numerous examples where the through route turns the corner, with the give way line crossing the 'straight on' alignment. A small proportion of T Junctions also use the Stop Line instead of a Give Way Line, this is generally where visibility is poor at the junction.

In the past, many simple T junctions were converted to a 'triangle junction' by splaying the side road either side of a small triangular island. This separated left and right turning traffic and also eased the corner for traffic turning into the side road from either direction. This design, once popular, has gone out of fashion as it is now considered safer to have a square approach to the junction, rather than requiring drivers to use their mirrors or look over their shoulders to check the road is clear. A more modern adaptation at the busiest T junctions has been to widen the main route to provide central right turn lanes, while the bellmouth on the side road is also widened to separate left and right turning traffic.

Crossroads

Main Article: Crossroads
A rural crossroads on the A22 on the Isle of Man

A Crossroads is found where two through routes cross each other. They are also very common, and were widely used in urban settings in the past, particularly during the Victorian era when Grid Iron street layouts were favoured. Priority is almost always given to the larger traffic flow, even if this means that an A road has to give way to a B or unclassified route. As with T Junctions, many have been signalised in recent years, and some have Stop Lines in place of Give Way Lines. Right turn lanes and segregation of turning traffic on the side road have also been introduced at many crossroads, where space permits. When the two routes cross at an acute angle, the junction is often referred to as a 'skewed crossroads'.

A common problem encountered in the past, particularly in rural areas, was that some traffic failed to slow down or stop for the junction. This lead to accidents, and junctions being branded 'Black Spots'. The simple solution carried out in numerous locations across the country was to stagger the junction by twisting one or more of the approaches away from the actual crossroads. This forced traffic on both side roads to slow or stop to safely make the turn onto the main route, before turning again to continue there journey. Right turn lanes, slip lanes and other features have often been added as well.

There are a small number of locations where more than four roads (two routes) meet. If five roads meet, the junction is often termed a Fiveways, and similarly six roads is termed Sixways. Both of these junction types are very rare, and the majority have been converted to roundabouts if space permits.

Roundabouts

Main Article: Roundabout
Olchfa Roundabout on the A4118 in Wales

Since the first roundabout in Great Britain was built at Sollershott Circus in Letchworth Garden City in 1909, they have become a common feature across our road networks. Their layout means that they can quickly, easily, and in most instances safely connect multiple roads together and allow traffic to make left or right turns with equal ease. The busiest roundabouts have been fully or partially signalised to better control traffic flows, and some have been converted into Hamburger designs, with roads built across the central island to try and speed up certain movements. There are numerous other variations to the general roundabout theme, some related to the shape of the central island and some with unusual priorities, where circulating traffic has to give way to a dominant flow in to the junction.

In urban areas, and to a lesser extent rural areas, numerous T Junctions and Crossroads have been converted to a roundabout format to try and improve traffic flows. Where space permits this has been done by inserting a very small traffic island, but in the most restricted locations a painted island is used to create a 'Mini Roundabout'.

While the term 'roundabout' is the most prevalent across Britain and Ireland, the terms 'Circle' or 'Island' are in common usage in some areas, such as the Midlands and Dundee area. In Scottish Gaelic a roundabout is termed 'Cearcail Rathaid', quite literally circle road. Developers and town planners sometimes also use alternative names, such as Circus or Oval.

Grade Separated Junctions

Main Article: GSJ

Simple designs

The majority of Grade Separated Junctions only provide freeflow for the busiest of the routes passing through the junction. Grade Separated Junctions were rare in Britain and Ireland before motorway construction began in the 1950s, and at that time the two most commonly used variants were the Diamond interchange and the Roundabout interchange. The former essentially uses a crossroads layout on the minor road, while the latter provides a roundabout to connect the slip roads together. Variations predominantly related to the omission of a sliproad for whatever reason. The Diamond layout had the benefit of only requiring a single bridge to be constructed, whereas the roundabout required two. In more recent years, the Dumbbell has been used in place of a Diamond, as it still only requires a single bridge, but by collecting the sliproads together at roundabouts, turning movements are made easier. In all cases, many of these junctions have been signalised over the years.

Freeflow Designs

A Trumpet on the M5
A Roundabout Interchange between two freeflow dual carriageway routes

Where two motorways meet, or indeed a motorway and another major route, a freeflow GSJ is the preferred solution, as this removes any conflicts between turning traffic. There are a variety of designs in use. When the junction only has three arms, a Trumpet Junction (left) or Directional T junction are the most common, the Trumpet only requiring a single bridge while the Directional T can require up to three bridges. Four arm layouts are more varied and include the following, some of which can be adapted to incorporate additional routes:

  • Cloverleaf Interchanges are generally disliked in the UK due to conflicts between joining and departing traffic
  • Four Level Stack
  • Whirlpool
  • Hybrid examples also exist, including Partially Unrolled Cloverleafs and Parclos.
  • In some cases, the two major routes cross with a roundabout provided as well to permit the turning movements (right) These can have some freeflow links for certain movements.

Lists of Junctions by County

Lists of Junctions by Authority





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