The modern A1 mainly follows the route of the Great North Road. This was a major coaching route in Britain and was used by the mail coaches between London, York and Edinburgh. The Great North Road, in part, followed the course of the Roman Ermine Street from Alconbury, as far north as Colsterworth (at the A151 junction).

An older and alternative route to the north from London was the Old North Road, now the A10 to Royston and then north to Huntingdon, avoiding Cambridge. This followed the initial section of Ermine Street and joined the Great North Road at Alconbury.

Adjacent to the eastern carriageway of the modern A1(M) at Alconbury Hill is a square pillar topped by a spire and ball and protected by iron railings. It marked the intersection of the Great North Road and the Old North Road and alternative routes and mileages to London, 65 miles on the Old North Road and 68 by the Great North Road,  are indicated on it by pointing hands. A third route is indicated via Cambridge. This presumably took the route of the modern A10 from Cambridge to Royston where it joined the Old North road. However it is possible it went by way of Great Chesterford  at which point it would have joined the Newparket Road. Until the late 1990s when this part of the A1 became a motorway, the milepost stood in the central reservation of the dual carriageway.

A traditional starting point of the Great North Road was the now demolished Hicks Hall at Smithfield in Central London. Milestones and distances in road atlases were measured from this point. The route ran from Smithfield up St John Street to the Angel Islington. However, with the building of the General Post Office at St Martin's-le-Grand in 1829, coaches started using the alternative route used by the modern A1, beginning at the GPO building and following Aldersgate Street and Goswell Road before joining the old route at the Angel.

In 1983 it was listed Grade II and described as follows: Late C18 limestone milepost. Square plan with plinth and moulded cornice, shaped cap with ball finial. Handscarved in shallow relief with inscriptions on two faces.

North-east face inscribed :
To London 64 miles through Huntingdon, Royston and Ware, To Huntingdon 5 miles; To London 72 miles through Cambridge, To Cambridge 21 miles, To Stilton 7 miles.

North west face inscribed:
To LONDON 68 miles through Buckden, Biggleswade, Hatfield, To Buckden 7 miles, To Stilton 7 miles. Post surrounded by railings with castiron fleur de lys finials.