From Roader's Digest: The SABRE Wiki
The 1922/3 Route
The A20 was first classified in 1922. It began then as it does now, on the A2 between New Cross Gate and New Cross. It followed the present route along Lewisham Way, passing Lewisham before heading towards Eltham. At this point, a gap was left in the 1922 route for the Eltham southern bypass which was under construction at the time.
The route resumed at the junction with Footscray Road, just to the west of Sidcup, and ran along the presnt A211 through Sidcup before following the present B2173 Maidstone Road through Swanley. The road briefly ran through the Swanley Interchange on the present A20 route before following the present M20 for a short distance. It then ran through the middle of Farningham before joining the present A20 route past Brands Hatch to Wrotham.
The route then followed the present A20 to Maidstone where it originally passed through the heart of the town centre along High Street and King Street before resuming the present route to Harrietsham where it passed through the village along West Street before rejoining the present route to Ashford. In Ashford the A20 ran along Maidstone Road and New Street to the Tank, then the High Street, East Hill and Hythe Road, rejoining the current route east of Willesborough.
The last leg of the original route followed the present A20 to Cheriton on the outskirts of Folkestone, where it then followed the B2064, A2034 and briefly the A2033 to the town centre, where a right turn took the A20 to its terminus on Guildhall Street, which is now closed to vehicles, and has pavement cafes!
The A20 has seen as many changes as its bigger brother the A2. Not long after classification was completed, the Eltham bypass and Sidcup Arterial Road were finished. A few years later, the small village of Farningham gained its bypass. The A225 has always been multiplexed so its southern end was moved to meet the new road. In 1927, Lenham gained its bypass, followed by Wrotham the following year.
The Motorway Era
Work finished on the Ashford Bypass in 1957, while the delay of the Maidstone Bypass allowed it to be built as a motorway, rather than the controversial at-grade roundabout with the A249 that was originally planned. It was completed in 1961 and the A20 though Maidstone was renumbered A2020. Seven years later, the Swanley Bypass opened and that allowed traffic to bypass the busy town. As the M20 was slowly being extended, the A2020 was renumbered back to A20 in 1974 and then in 1976, the Swanley Interchange was fully rebuilt to commutate the M20 and M25. Although the overbridge wouldn't be used for ten years, it didn't affect the junction that much.
With the opening of the Wrotham Hill Interchange in 1980, the section of A20 from Swanley to Maidstone would lose most of its traffic apart from raceday at Brands Hatch. As the British Grand Prix used to be hosted at Brands in even-numbered years, it was ultra busy but after 1986, the Grand Prix went to Silverstone and the A20 was never that busy again. In 1981, the eastern section of the M20 opened, part of it on the Ashford Bypass. There was still a ten mile gap on the M20 and the A20 took the brunt of the traffic between Maidstone and Ashford.Channel Tunnel that saw the biggest changes on the A20. The Sidcup bypass was upgraded to D2 and a new section, the Ruxley bypass open to link it to the Swanley Bypass. The M20 between Junction 8 and Junction 9 was built to take the strain off the A20 and an all new section was built between Folkestone and Dover containing a impressive tunnel though one of the hills and great views. All the works for the roads were finished in 1993. Since then there have been no major works on the A20, however its junction with the A228 was upgraded when the A228 became D2 in the late 00's.