The United Kingdom's roads are arranged radially around London and Edinburgh. A-roads with single-digit numbers form the main arteries:
  • The A1 is the Great North Road from London to Edinburgh along the east coast.
  • The A2 runs south-east from London to the channel ports.
  • The A3 runs more or less south from London.
  • The A4 is the main road west from London towards Bath.
  • The A5 is Watling Street, the main road north-west from London to Holyhead.
  • The A6 is the road to Carlisle.
  • The A7 is the continuation of the A6 north to Edinburgh.
  • The A8 runs west from Edinburgh to Glasgow and thence to Greenock.
  • The A9 leads north from Edinburgh to Scrabster, near Thurso.
Roads on the clockwise side of a main A-road have two, three and four digit numbers beginning with the appropriate digit, so that near the A1 you find the A10, the A1000 and the A11. Between the A1 and the A7 in Scotland, the roads are treated as being clockwise of the A6.

B-roads are more minor, and similarly grouped. Motorways are usually numbered to match nearby A-roads, which leads to incongrous results as the motorways are extended.

Twinned with How the United States Highway System Works.

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