Dual carriageway

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Dual carriageway
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A typical rural dual carriageway, in this case the A449.
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A dual carriageway (or dual-carriageway road) is a road where there is a physical barrier separating the two directions of travel. They first appeared on the British road network in the 1920s and became popular from the 1950s onward. Various acronyms are given to the standard of a dual carriageway, which may range from D1 (one lane each way) to D5M (five lanes each way, plus hard shoulders). Although the majority of motorways are dual carriageway, simply having three lanes and a hard shoulder does not make a road a motorway.


The A27 near Havant has four lanes of traffic in each direction plus hard shoulders (ie: D4M). However, this does not automatically make it a motorway.

In order to be a dual-carriageway road, there must be more than a painted line separating the directions of travel. Despite a common misconception, a dual carriageway is not related to the number of lanes – whilst the most common form of non-motorway dual carriageway is that of two lanes per direction of travel, there are many examples where there are more than two lanes; as well as an increasing number of roads where there is only a single lane of traffic per direction of travel. The latter roads are often where a dual-carriageway road has been hatched down with paint or other barriers to a single lane per direction, usually with safety given as the reason for doing so. The A449 between Worcester and Kidderminster is a well-known example of this type of road. Other examples are built and known as single-lane dualling from the outset, usually in the form of an extended traffic island to protect and assist right-turning traffic into and out of a side road.

Most motorways are created with a dual-carriageway layout. Most motorways were built as dual 2 or 3 lane with hard shoulders, but many major stretches have been widened since. One example is the Preston Bypass, the first motorway stretch that is now part of the M6. When opened, the road was D2M (see below for explanation), but was built with an especially wide central reservation for future widening to D3M, which took place a few years later. However, in the 1990s, the road was widened again to D4M, and the original road is now almost completely unrecognizable. Another more recent example is the M4 between the A312 and the A308(M)/[A404(M)], which was originally built as D2M, then widened to D3M in the 1970s, and now work is underway to add even more capacity by converting the road to D4ALR (all lane running) between J3 and J12.

In the United Kingdom, the national speed limit has been higher on a dual-carriageway road than on a single-carriageway road. When speed limits were raised in the late 1970s, following a temporary reduction to deal with the 1973 Oil Crisis, the number of high-quality dual carriageways in existence made it sensible for these to be assigned them a limit comparable to that applied on motorways. This means that the Llywel Mountain Road in Brecknockshire can be driven legally by a car at 70 mph, though it would be foolish to do so. However, many urban or accident-prone dual carriageways have a reduced speed limit, usually 50 mph or less. During roadworks temporarily reduced speed limits are put in place on dual carriageways in the same way as they are on motorways.

Although it is still uncommon to see variable speed limits on non-motorway dual carriageways, electronic lane control signals on a few motorway-like stretches (such as the A2 between the M25 and M2 and the A282) can set an advisory reduced speed limit, and the A14 between Cambridge and Huntingdon is the first smart A-road stretch in England to use variable mandatory speed limits like smart motorways.


The dual-carriageway A33 Winchester Bypass under construction in 1933

Whilst there are a number of historical roads that were originally made dual carriageways for ornamental reasons or to allow traffic to pass a marketplace, the practice of building of them to allow for greater traffic flow dates from the 1920s. These early city-based dual carriageways within cities – such as Princess Road (now the A5103) and Kingsway (now the A34) in Manchester, which date from 1925 and 1926 respectively – often initially carried a tramway in the central reservation.

Following the initial urban dual carriageways, from the mid-1930s bypasses, such as the one at Winchester, began to be constructed as dual carriageways from the outset. However, many roads constructed in that era that are today dual carriageways – such as the A3 Guildford and Godalming bypass, the A4123 Wolverhampton to Birmingham New Road, the A580 East Lancashire Road, and the A127 Southend Arterial road were actually constructed as multi-lane single-carriageway roads before later widening.

Whilst the location of the first dual carriageway in England, Wales, or Ireland has yet to be established, it is though that the first dual carriageway in Scotland was the A823 Queensferry Road between Dunfermline and Rosyth.

A classic piece of online dualling - the A1 north of Wetherby. The road has now been replaced by the parallel A1(M).

Adding a second carriageway to an existing, formerly single-carriageway, road is known as online dualling. This become popular following World War II as it provided an easy way to increase a road's capacity while being relatively cheap to construct if the second carriageway could be built on land already owned by the highway authority. Large sections of the A1 and the A74 were improved in this way in the 1950s and 60s, simply by putting a second carriageway down. Online dualling was done simply where it was practical, so it was common to have a small stretch of dual carriageway come to a halt with narrow single-carriageway sections either side, where the land was privately owned or the geography made dualling impractical. This included parts of the former A3, the A12, and the A64 and is still a problem on the A303, most obviously where the dual carriageway westwards stops due to the historically important land surrounding Stonehenge.

A dangerous at-grade right turn on the dual-carriageway A1 in Lincolnshire. For safety reasons, these are increasingly being grade-separated or the gap is closed.

No thought was generally given in online dualling to the safety or practicality of the junctions with adjoining side roads, which in many cases remained unchanged, leading to traffic having to turn right across one lane of 70 mph traffic into another one. Consequently, many of these turnings became accident blackspots and have either been closed off or remodelled to give proper grade separation. Early design standards also did not consider grade separation; the Hockley Interchange, a flat crossroads connecting the dual-carriageway A33 to the single-carriageway A333 near Winchester was considered one of the worst bottlenecks on the British road network before it was replaced by the M3 over Twyford Down and demolished.

Lane designations

In the UK the lanes are designated with the far-left lane (next to the hard shoulder, if there is one) being referred to as lane 1, with the number increasing sequentially for each lane moved to the right. Colloquially, these are sometimes (and incorrectly) referred to as the slow lane and overtaking lane (or fast lane).

All lanes except for lane 1 are designed for overtaking traffic in normal conditions, never simply because a vehicle is perceived as "fast", and this is backed by rule 238 in The Highway Code. An exception is in queues, where traffic should use all available space unless directed otherwise (e.g. by signs). Epithets such as "middle lane hog" (referring to drivers who stick to the middle lane of three without any regard to other vehicles or to the general flow of traffic) have become well-known, and driving in this way became a traffic offence punishable by on-the-spot fines in 2013.

Carriageway designations

In England, most trunk road carriageways have individual references (A or B, according to direction, plus some others listed below) as an aid to police, breakdown staff, and road planners and workers. These designations are prominently displayed on the Driver Location Signs provided on many dual carriageways by National Highways.

Standard Designated Letters

  • A = Junction numbering count increases, e.g clockwise, away from London
  • B = Junction numbering count decreases , e.g anti-clockwise, towards from London
  • C = Secondary "A" Carriageway
  • D = Secondary "B" Carriageway
  • J = Off-slip from "A" Carriageway
  • K = On-slip from "A" Carriageway
  • M = Off-slip from "B" Carriageway
  • L = On-slip from "B" Carriageway

The signs are dictated by the rules set out in 193 of the DMRB

Note: there are some contradicts to these rules, such as the M55 where the junction numbers run west to east, but the A/B designation is the opposite way around. There are also nuances on more complex junctions, such as the M4/M25 Interchange.

Dual carriageway types and acronyms

With the introduction of Smart Motorways which convert the hard shoulder into an extra lane (either dynamic hard shoulder or all lane running), sometimes the letters 'SM' or 'ALR' are added to the end to illustrate the smart motorway or all lane running where there is no permanent hard shoulder, if there is one at all.

All-purpose smart roads, so far only the A14 between Cambridge and Huntingdon, may also use the SM or ALR suffix, however Dual Carriageways that have lane control for only short parts are not, such as where lane control exists through tunnels or over bridges.


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D1 refers to a road with a single lane in each direction, separated by a central reservation. D1 roads do not have a hard shoulder, and are usually formed when a wider road has had paint applied to it to turn from two lanes per direction to one for safety reasons. However, there are some short purpose-built sections, often where there is an obstacle in the route, or to accommodate the local lie of the land.


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D2 refers to a road with two lanes in each direction, separated by a central reservation, but with no hard shoulder. D2 roads are often what is colloquially meant when a layperson uses the phrase 'dual carriageway', and they are the most common type of dual carriageway across the United Kingdom and Ireland.


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D3 refers to a road with three lanes in each direction separated by a central reservation, again with no hard shoulder. D3 roads are often misunderstood by the public and incorrectly referred to as being motorways. However, some All Lane Running schemes use D3ALR, for example the A14 between Cambridge and Huntingdon (the first non-motorway smart road), and the proposed conversion of the A1(M) between J6 and J8 from D2M to D3ALR.


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D4 refers to a dual-carriageway road with four lanes in each direction separated by a central reservation, with no hard shoulder. Confusingly, this layout appears both on all-purpose roads, such as the A13, and on smart motorways with all-lane running, such as parts of the M25 and M62, where the layout is sometimes called D4ALR. On dynamic hard shoulder schemes, such as parts of the M6 and M42 in the 'Birmingham Motorway Box', the standard can be changed; these roads are normally D3M but could be said to be D4 when the hard-shoulder running is active.


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D5 refers to a dual-carriageway road with five lanes in each direction separated by a central reservation, with no hard shoulder. These are rare and short, mostly occurring in between junctions. Two examples exist on the A406 North Circular Road, between the M11 and A104, and between the A41 and M1 junctions.


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D2M is a term for a dual-carriageway road with two lanes and a standard width hard shoulder in each direction. Despite the use of "M" in the acronym, it does not imply the road necessarily has motorway restrictions, and the term can be applied to all-purpose roads as well, although D2M all-purpose roads are rare and are usually former motorways, such as the A41 Tring bypass (formerly the A41(M)) and the A414 St Albans bypass (formerly the M10).

An alternative, but seldom used, way of writing the abbreviation is D2(M).


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D3M is a term for a dual-carriageway road with three lanes and a standard width hard shoulder in each direction; and is the standard design for motorways in much of the UK. The total width of such a road, with six traffic lanes, two hard shoulders, a central reservation, and verges to the side is approximately 32 m (105').

There are some all-purpose roads of this type, most notably parts of the A13.


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D4M is a term for a dual-carriageway road with four lanes and a standard width hard shoulder in each direction. Despite the use of "M" in the acronym, it does not imply the road necessarily has motorway restrictions, and the term can be applied to all-purpose roads as well; for example, the A2 is built to D4M standard between the M25 and the M2.

In England, D4M is only used for roads with daily flows of over 70,000. In other countries this width of motorway would be justified at much lower traffic levels.


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D5M is a term for a dual-carriageway road with five lanes and a standard width hard shoulder in each direction. This is quite a rare type, and the first example within the United Kingdom was on the M2 in Belfast.


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D6M is a term for a dual-carriageway road with six lanes and a standard width hard shoulder in each direction. There is only one example of this within the United Kingdom, the M25 section at Heathrow Airport (Junction 14 to Junction 15).

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