From Roader's Digest: The SABRE Wiki
The classification of roads varies throughout the British Isles. Although originally done as a means of recognising the level of funding for maintenance of a road, it is more commonly used for ease of navigation, and is an important component of direction signs.
In Britain, the all-purpose roads are grouped into nine Zones, with the majority of the boundaries being the single digit roads. The exception is the boundary between zones 1 and 2, which is the River Thames rather than the A2. There are two hubs to the system, with A1–A6 radiating from London and A7–A9 radiating from Edinburgh.
Route numbers can have from one to four digits in the case of "A" class roads, and three or four digits in the case of "B" class roads. These roads form one system across Great Britain, and there should be no duplicates. This is, however, not always the case and some duplicates do exist. In addition, numbers can be recycled and can be applied to a different route from the original – usually with a time delay between one use and another.
The classification of the roads in Britain was first completed in 1922, although there have been many changes over the years, and there was a major revision in 1935. The A1 formerly used part of the present route of the A19, whilst the A66 had a route that headed into York. The first renumbering due to a building of a bypass is believed to be the first Dartford Bypass, built in 1924 which became part of the A2.
In 1964, Primary routes were introduced, being recommended routes between places "of major traffic importance", usually large towns. Primary routes have direction signs with green backgrounds and white lettering, whereas all other non-motorway roads have direction signs with white backgrounds and black lettering. Individual A-class roads may be partially primary and partially non-primary.
Other important routes within Highway Authorities are given internal classifications within each authority which are not meant to appear on signage. These so-called "C" class roads may actually be allocated any letter code ("C", "D" or "U" being examples), and the numbers can be reused in different authorities.
In 2011, the Department for Transport consulted on reforming the administrative procedures in place regarding route numbering on the non-trunk network. These changes when implemented in 2012-13 will make it easier for local authorities to renumber roads to reflect strategic need. It is not yet known how extensive the safeguards to prevent any anomalies between local authorities will be, but it has been suggested that cross-boundary issues will be referred to the Department for a decision.
In 1958, the Preston Bypass opened and the motorway era began, although the numbering system was not finalised until the 1960s.
Motorways also follow a zonal system, which is different between Scotland and England & Wales, and are given a number either prefixed with M, or suffixed with (M). Direction signs on motorways have a blue background with white lettering.
Scottish motorways are numbered according to the number of the "A" road that they replace, hence the reason there is no M7 – it has not been necessary to build a motorway along the line of the A7.
In England and Wales the motorway zones are different, though they again use the single digit motorways (plus theoretical extensions thereof). The boundary between the motorway zones 2 and 3 consists of the M3 to junction 8, then a theoretical straight-line extension southwest from J8 to the coast of Cornwall. The M3 south of junction 8 was not part of the originally planned motorway, and so does not form the zone boundary.
Motorways can also be given a suffix of (M). These roads, whilst being full motorways, are used for shorter bypasses of sections of "A" class roads and are so numbered in order to preserve the long-distance route number.
Changes to zones
It is unknown as to whether the zone boundaries move when a single digit road is rerouted. Some renumberings (such as those regarding the A1 in the Tyneside area have caused a mass of other renumberings (for example, from A1(M) to A6127(M) to A167(M) as the zone boundaries moved.
Other renumberings of single digit roads have not seen such wholesale changes.
Road numbers are used less frequently for navigation in Northern Ireland compared to Britain. The roads don't follow a zone pattern, though some numbers can reflect allocation based on geography.
The A1 follows the historic Belfast - Dublin postal route, while the A2 follows the coast road around Ulster from Newry to Derry. The A3 to A6 inclusive then took the next 4 most important routes, increasing in number in a clockwise direction.
The A20 to A29 begin south/east of the A1-A3 axis, increasing in a clockwise-around- and distance-from-Belfast sequence. The A7 and the Cullaville version of the A37 are in this area. The A30 to A35 follow a similar pattern between the A3 and A6 and the A8, A36 and A37 north of the A6. The A38 and A39 took short links in the area between A3 and A6. The A40 was an early addition, and the A42-A44, A45-A47, A48-A51 and A52-A54 form four sequences of very early additions. The A57 and A76 were numbered after the B roads that they were upgraded from. The A55 took the lowest available number when given a two-digit number.
The A500 and A501 were allocated to very short links in Belfast, joined later by other A50x routes. The A5xx sequence broke out of being Belfast with the A505, and they stopped being allocated sequentially.
Republic of Ireland
In the Republic of Ireland, road classification happened just after partition and their original classification was very different to that of the UK. Originally the roads were designated "T" for trunk roads and "L" for link roads, but these were generally not as well known as numbers in Great Britain, and did not receive official status from the government. The lower numbered T roads formed radials from Dublin to other major towns and cities, while subsequent numbers were allocated to other cross country routes, generally in order of decreasing importance.
A comprehensive review was undertaken in the late 1960s, and formally implemented as a major reclassification of roads in 1977, introducing the "N" system. Some of the "T" roads became the current "N" roads. For example, T1 is now the N1, and T2 is now the N2. Unlike "T" roads, "N" roads have legal standing and are formally recognised.
All the "N" roads are numbered in a anti-clockwise fashion from Dublin. So N1 is at the top, N11 at the bottom. Then it starts again in the same way. There were originally only 25 Primary "N" roads. When the Nass bypass opened in 1983, that became the first motorway in the Republic of Ireland. Motorways in Ireland are part of the N-Road system so as the Naas Bypass was part of the N7, it became the M7, though this purely for signing convenience - it legally remains part of the N7.
The first primary "N" road classified after the original twenty-five was the N26 in County Mayo. The last "N" road classified so far in this sequence was the N33 in 2004 which supposes that next number in the scheme will be N34. However there are some oddities, the biggest being the N50 being numbered the N50. As it all motorway, it is signed M50.
Non-primary "N" roads have numbers over fifty. As in the UK, primary routes have green background signs, motorways have blue background signs and all other routes have white background signs.
In 1994, regional "R" roads were recognised by the government. "R" roads tend to radiate anticlockwise from Dublin around the entire country, though this is only a loose pattern, and are mostly grouped together within the first digit, eg. all the R1xx roads are all near to each other.
"L" road numbers were revived in 2005 but for a different purpose to the original scheme. "L" roads are all the remaining roads in the Republic of Ireland that are not "N" roads or "R" roads. Many "L" roads have numbers five digits long.
Isle of Man
On the Isle of Man, there are and (and C, D, E and U roads). A1, A2, and A3 formed a triangle linking the largest towns on the island: Douglas, Peel and Ramsey, with the A4 forming a N-S route down the middle of the island, connecting the other town, Castletown. This has been changed, with the A3 and A4 swapping.
The A5 forms the main route from Douglas to the south east corner of the island. The A6 formed a branch to Douglas' docks (now diverted and extended to form a loop around Douglas), the A7 and A8 being two spurs into Castletown - one from Port Erin, the other from Ballasalla. The A5 has since been rerouted via Castletown and the Ballasalla to Port Erin route has become a recycled A7. The A8 has been recycled as a short road in Douglas town centre.
2-digit A roads cluster together, though there are some exceptions:
- A1x: North of the Island (A12 is, however, on the south coast)
- A2x: Middle of the Island
- A3x: South of the Island (A35, A38, A39 in Onchan, A33 recycled in Douglas)
- A4x: Douglas (A40 in St Johns, A48 and A49 don't exist)
B Roads are numbered roughly north to south with higher numbers used for newer routes - most of which are in Douglas.