From Roader's Digest: The SABRE Wiki
|A typical rural dual carriageway, in this case A449.|
|Pictures related to Dual carriageway|
View gallery (4)
|Motorway • High Quality Dual Carriageway • Expressway|
A Dual carriageway road is a road where there is a physical barrier separating the 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 four lanes and a hard shoulder does not make a road 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 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. 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.
In the United Kingdom, the national speed limit has been higher on a dual carriageway road than a single carriageway road. When speed limits were raised in the late 1970s following a temporary reduction to handle the 1973 Oil Crisis, the number of high-quality dual carriageways made sense to assign them a comparable limit to motorways. This means that the Llywel Mountain Road in Powys, Wales can be driven legally by a car at 70mph, though it would be foolish to do so. However, many urban or accident-prone dual carriageways have a reduced speed limit, usually 50mph or less. Similar speed limit reductions are normally in place during roadworks, like motorways.
Whilst there are a number of historical roads that were dual carriageways for ornamental reasons or allowing for traffic to pass a marketplace, the building of them to allow for greater traffic flow dates from the 1920s. These early city-based dual carriageways within cities often initially carried a tramway in the central reservation, such as Princess Road (now A5103) and Kingsway (now A34) in Manchester which date from 1925 and 1926 respectively.
Following the initial urban dual carriageways, bypasses started to be constructed as dual carriageways from the beginning in the mid-1930s, such as the Winchester Bypass. However, many roads constructed in that era that are today dual carriageways such as the A3 Guildford and Godalming bypass, A4123 Wolverhampton - 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 it is not conclusively known at this time as to the location of the first dual carriageway in England, Wales or Ireland, the first dual carriageway in Scotland is thought to be the A823 Queensferry Road between Dunfermline and Rosyth.
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 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 was impractical. This included parts of the former A3, A12 and 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.
No thought was generally given to the safety or practicality of adjoining side roads in online dualling, which in many case remained unchanged, leading to traffic having to turn right across one lane of 70mph 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.
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 enforced by rule 238 in the Highway Code. An exception is in queues, where traffic should use all available space unless directed otherwise (eg: by signs). Epithets such as the self-described "middle lane hog" have become well-known among road enthusiasts, and became a traffic offence punishable by on-the-spot fines in 2013.
In England, most trunk road carriageways have individual references to aid police, breakdown staff, and road planners/workers. Typically, the main carriageways are labelled A and B, though this is not always the case. The references are prominently displayed on many Highways England roads using Driver Location Signs.
Dual carriageway types and acronyms
|Pictures related to D1|
View gallery (27)
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 account for the local geology.
|Pictures related to D2|
View gallery (1043)
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.
|Pictures related to D3|
View gallery (122)
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.
|Pictures related to D4|
View gallery (25)
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 on both All-purpose Roads such as A13, and Smart Motorways with All Lanes Running, such as parts of M25 and M62. On Dynamic Hard Shoulders portions of Smart Motorways such as parts of M6 and M42 in the West Midlands conurbation, the standard can be changed; these roads are normally D3M but could be said to be D4 when the hard shoulder running is active.
|Pictures related to D2M|
View gallery (610)
D2M is a term for a dual carriageway road with two lanes and standard width hard shoulder in each direction. Despite the use of "M" in the acronym, it does not mean the road has to have motorway restrictions, and 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 A41(M)) and A414 St. Albans Bypass (formerly M10).
An alternative, but seldom used, way of writing the abbreviation is D2(M).
|Pictures related to D3M|
View gallery (392)
D3M is a term for a dual carriageway road with three lanes and standard width hard shoulder in each direction; and is the standard width 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 32m (105').
|Pictures related to D4M|
View gallery (68)
D4M is a term for a dual carriageway road with four lanes and standard width hard shoulder in each direction. Despite the use of "M" in the acronym, it does not mean the road has to have motorway restrictions, and can be applied to all purpose roads as well; for example, the A2 is built to D4M standard between the M25 and 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.
|Pictures related to D5M|
View gallery (4)
D5M is a term for a dual carriageway road with five lanes and 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.