The original A1 route was designated by the Ministry of Transport in 1921, following the medieval Great North Road from Central London through Barnet, Potters Bar, Hatfield, Welwyn, Stevenage, Baldock, Biggleswade, Sandy, St Neots and Alconbury, then joining the route of a Roman road, Ermine Street, as far as Colsterworth. The route was modified in 1927 when bypasses were built around Barnet and Hatfield.
Continuing north, the A1 runs on modern bypasses around Grantham, Newark-on-Trent, Retford, Bawtry, Doncaster, Scotch Corner, Chester-le-Street, past the Angel of the North sculpture, around Newcastle-upon-Tyne, Morpeth, Alnwick, Berwick-upon-Tweed, into Scotland, past Dunbar and Haddington before finally arriving in Edinburgh.
Some sections of the A1 have been upgraded to motorway standard. These are known as the A1(M). These include a long stretches coming north from London, the new Peterborough section, a short stretch either side of the M18 junction, and another long stretch south of Newcastle-on-Tyne.