From Roader's Digest: The SABRE Wiki
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 & M25. Although the overbridge woundn'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, 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.