The first section begins on Old Market Street near the centre of Bristol and passes through Kingswood before leaving the city on the east side.

It then travels under the Great Western Main Line at the twin-arch Acorn Bridge (the second arch was originally used by the Wilts & Berks Canal) and past Shrivenham and Watchfield (both bypassed in the 1980s), then on towards Faringdon in the Vale of White Horse.

A further by-pass section, opened in 1979,[1] avoids the centre of Faringdon, passing just south of Folly Hill and crossing the A417.

The A420 then travels the corallian limestone ridge that forms the north-west boundary of the Vale of White Horse, passing Littleworth, Buckland and Longworth.

A further dual-carriageway section bypasses Cumnor Hill, to give a view of the "City of Dreaming Spires" that is Oxford from the west.

As a result of the building of the M4 motorway in the early 1970s and subsequent road modernisation, the A420 between Chippenham and Swindon lost its identity.

On 27 January 2007, Timothy Brady, a 33-year-old man from Harrow, London, was clocked driving at 172 miles per hour (277 km/h) in a Porsche 911 Turbo that he had taken without permission from his employer, a luxury car hire firm.