|West end:||Old Market Street, Bristol|
|East end:||Headington in Oxford|
Since the opening of the M4 motorway, the road is in two sections. The first section begins on Old Market Street near the centre of Bristol, it passes through Kingswood before leaving the city on the east side. From here it travels eastward over the southern part of the Cotswolds, to the north of Bath, to Chippenham in Wiltshire.
The second section starts at a junction with the A419 east of Swindon. It then travels under the Great Western Main Line at the twin-arch Acorn Bridge and by-passes Shrivenham (the road originally went through Shrivenham, but the by-pass was built in the mid-1980s) and Watchfield, then on towards Faringdon in the Vale of White Horse. A new section by-passes Faringdon, just south of Folly Hill and crosses 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 dual-carriageway section by-passes Southmoor and Kingston Bagpuize on its way to Oxford. Most of the road between Swindon and Oxford, apart from the dual carriageway sections and a short section around Faringdon, is limited to 50 mph (80 km/h).
A further dual-carriageway section bypasses Cumnor Hill, to give a view of the "City of Dreaming Spires" that is Oxford from the west. It then passes the Oxford Ring Road, through the suburb of Botley and down the Botley Road. It crosses the River Thames on Osney Bridge and reaches central Oxford after passing under the Cherwell Valley Line next to Oxford station. Within Oxford, it is routed along Oxpens Road, then Thames Street, parts of St Aldate's then the High Street, which is closed to most motor traffic. The road then crosses Magdalen Bridge to St Clements and East Oxford and ascends the notoriously steep Headington Hill to the suburb of Headington before terminating at the Green Road roundabout, where it meets the A40 and the Oxford Ring Road.
Large vehicles such as lorries are advised by large signs at Oxford and Swindon not to take this route and use the alternative A34 and M4 route to Swindon. This is not enforced and the road is often heavily congested due to slow HGVs taking the shorter A420 route. The road has a poor accident record and this coupled with heavy peak time traffic has caused it to be nicknamed the 'A420 Road to Hell' in the local media.
What is now the A420 was established in the early 18th century as a direct route between Chippenham and Bristol via Tog Hill, avoiding the older and more established route via Bath (now the A4). It was an important road for Bristol, whose communications with Bath had been disputed and difficult. After the Bath Road was turnpiked in 1707, the Bristol Trust attempted to turnpike the direct road in 1727, but faced opposition from colliers at Kingswood, and the road was not sufficiently improved until the 1740s.
When first classified in 1922, the A420 ran between Chippenham and Botley, near Oxford. The road between Bristol and Chippenham was the A430. The two roads were later joined and the A430 renumbered A420. The road from Botley through Oxford to Headington was originally part of the A40. When the northern Oxford bypass was built in the 1930s, the A40 was rerouted along the bypass and the road through Oxford was renumbered A420.
As a result of the building of the M4 motorway and subsequent road modernisation, the A420 between Chippenham and Swindon lost its identity. From Chippenham to Lyneham through Sutton Benger and to the north of RAF Lyneham and its limestone ridge, it became the B4069. From Lyneham to Swindon it became part of the A3102. From Swindon through Stratton St Margaret to the A419 it became the A4312.
Site of fastest speeding ticket
The A420 near Kingston Bagpuize in Oxfordshire was the site of the fastest speeding offence ever caught in a routine speed check in the UK. 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. Brady was disqualified from driving and sentenced to 10 weeks in prison.
- Email from Oxfordshire County Council Highways, 23 October 2007
- Gilbert, Dominic (30 August 2014). "A420 reopens with roadworks completed a week early". Swindon Advertiser. Retrieved 5 November 2015.
- Ffrench, Andrew (11 April 2014). "Councils consider a new safety plan on the dangerous A420". Oxford Mail. Retrieved 5 November 2015.
- Buchanan, Brenda. "The Great Bath Road 1700-1830" (PDF). University of Bath. Retrieved 10 August 2016.
- 1922 road list
- Kinross, John (1998). Discovering Battlefields of England and Scotland. Shire. p. 98.
- "Jail for 172mph Porsche motorist". BBC. 24 September 2007. Retrieved 16 January 2011.