The maintenance and development of all 25,000km of public roads in Northern Ireland falls under the responsibility of the Roads Service, which is part of the Department for Regional Development, set up under the Good Friday Agreement in 1998. At the root level, roads are divided into two types:

• Strategic or Trunk Roads - key transport corridors between major urban areas - 1,300 km
• Local Roads - all other roads - 23,700 km

Strategic Roads are themselves divided into two categories:

• Motorways - high speed, non-stop routes restricted to certain types of vehicle. There are a total of 115km of this type of road. Further details can be found here.
• Non-motorways - other major roads, ranging from dual-carriageways to single lane roads. There are about 1,200 km of trunk roads, a list of which is given below.

Motorways are generally assigned their own numbers either prefixed with the letter M or appended with (M), for example M1, M22, A8(M). Trunk roads as a whole consist of a number prefixed with the letter T and run in a sequence from T1 to T23 (see list below). T numbers are used by planners but never appear on signs (although you can see them on old OS maps from the 1960s).

All non-motorway roads, including the trunk roads, in Northern Ireland belong to one of 4 categories:

• A-class routes, which form the core of the road network. These routes are labelled with the letter A followed by up to three digits, eg A1, A27, A515. It is important to note that whilst ALL non-motorway trunk roads are A-class, NOT all A-class roads are trunk roads. It is also important to note that the numbers are route numbers, not road numbers. It is possible for an A-class route to turn off a road onto another road at a T-junction because they are descriptions of a route between two places, not a specific road between the two places. For example, the road past Shaw's Bridge in Belfast is the A55, but the A55 turns left onto Balmoral Avenue part way along the road. A list of the sets of A-class numbers in use is given below.
• B-class routes typically form the remainder of inter-town and inter-village links. Smaller towns and villages may only have B-road connections. These routes are labelled similarly to A-routes except that they are prefixed by the letter B.
• C-class routes are the remaining through roads in towns as well as most of the roads in the countryside. Although they do have numbers, like A and B routes but prefixed with a C, these generally do NOT appear on road signs.
• Unclassified roads paradoxically actually do have numbers, prefixed by a U, but these are only used by road maintenance workers and local government and never appear on signs. U-roads include most housing estates and private lanes.

The relationships between all these categories is best illustrated by this table:

A-class numbering in Northern Ireland

There is not much logic to the numbering of A-roads in Northern Ireland, but it follows the below pattern. Click here for a complete list of A-roads in Northern Ireland.

• A1 to A54 were apparently the originals, assigned when the system was introduced. There are a few blanks in this sequence.
• A501 to A523, again with a few blanks, were added later. Why this sequence was chosen instead of just continuing the two-digit numbers is a mystery.
• A55 was allocated to the Belfast Outer Ring to give it a complete number. It was previously numbered in parts, eg the Newtownbreda stretch was once A504.
• A57 (Belfast International Airport to Ballynure via Templepatrick) and A76 (Lurgan) were upgrades of the B57 and B76 respectively.
• A101 links the M1 to the A1 at Sprucefield. It was opened in 2003.
• A211 is Comber main street, formerly part of the A21 and given this number in 2004 after the Comber bypass was built.
• A371 is in Limavady, formerly part of the A37 and given this number in 2004 after the Limavady bypass was built.

These last three numbers are the most recent and appear to mark a shift in road numbering policy, as all three appear to be based on the number of the A-road they join.

According to a Northern Ireland Assembly written answer from 1999, the reason for the above system is lost to history: "Road classification dates back a very considerable time (pre Local Government Reorganisation) and its use today is limited to route identification. Any new road outside a motorway would be classified on the basis of the comparable route classification in the area. Roads Service have been unable to find any historic record of how the classification system was determined originally but it has no relevance to current funding allocations."

List of Trunk routes in Northern Ireland

The trunk routes are basically a subset of the above A-class roads, representing those that are most important. According to a list compiled by David McCormick and published here in 2002, they are as follows:

• T1 - A2 Belfast-Holywood-Bangor
• T2 - A24/A2 Belfast-Carryduff-Newcastle
• T3 - A1/A3/A4/A5 Belfast-Lisburn-Portadown-Ballygawley-Omagh-Derry
• T4 - A1 Belfast-Lisburn-Banbridge-Newry-border
• T5 - A3 Portadown-Armagh
• T6 - A5/A4 border (south of Ballygawley)-Ballygawley-Enniskillen- border (Belcoo)
• T7 - A6/A26/A37/A2 - Belfast-Antrim-Coleraine-Derry
• T8 - A26/A6/A31/A29 - Moira (A3)-Antrim-Castledawson-Cookstown
• T9 - A8 Belfast-Larne
• T10 - A509/A32/A505 - border (Belturbet)-Enniskillen-Omagh-Cookstown
• T11 - A20 Belfast-Newtownards
• T12 - A22/A21 Dundonald-Comber-Newtownards
• T13 - A7 Carryduff-Downpatrick
• T14 - A55 Belfast Outer Ring
• T15 - A28/A29 Newry-Armagh-Dungannon-Cookstown-Coleraine
• T16 - A27 Newry-Portadown
• T17 - A6 Castledawson-Dungiven-Derry
• T18 - A44 Clogh Mills (A26)-Ballycastle
• T19 - A36 Ballymena-Kilwaughter (A8)
• T20 - A57 Belfast Intl Airport-Templepatrick-Ballyclare-Ballynure (A8)
• T21 - A2 Greencastle-Carrickfergus, apparently incorporating the M5!
• T22 - A514/A515 Crescent Link-Foyle Bridge-Skeoge Road, all in Londonderry
• T23 - A46 Enniskillen-Belleek