Part of the route now followed by the modern road, particularly the western section, was opened as various turnpikes in the 18th century in an effort to improve coaching links between London and the Kent towns.

In the early days of the Great Britain road numbering scheme the A20 ran through Eltham.
0.7 miles (1.1 km) along Eltham Road the Sidcup Arterial Road begins, opened in 1923, which carries traffic south of the two towns instead, leaving the A210 and A211 roads following the original route.
 The nearby town of Swanley was bypassed in 1968, and the short link between the two bypasses was constructed in 1988 (to the south of the Ruxley Corner roundabout).

A section of the A20 in Ashford formed part of the Ashford By-Pass, a dual carriageway opened in 1957, which used to run from what is now the roundabout with Simone Weil Avenue to the Willesborough roundabout.
Simone Weil Avenue is the original A20 bypass, but has been diverted to curtail at Canterbury Road.
The eastern end of the old by-pass is now the M20.

The section between Maidstone and Ashford was the only link between the two separate sections of the M20 for 10 years during the 1980s, until the 14-mile (23 km) missing link of the motorway was completed in May 1991.

North of Folkestone the M20 becomes a dual carriageway and enters the Roundhill Tunnel before crossing over and dropping down into Dover.
This part of the A20 was completed in 1993 as a project included in the Parliamentary Bill for the Channel Tunnel.
During 2011, structural failures in and around the Roundhill Tunnel have caused the A20 to be closed in this area numerous times.
Part of the route now followed by the modern road, particularly the western section, was opened as various turnpikes in the 18th century in an effort to improve coaching links between London and the Kent towns.
In the early days of the Great Britain road numbering scheme the A20 ran through Eltham. 0.7 miles (1.1 km) along Eltham Road the Sidcup Arterial Road begins, opened in 1923, which carries traffic south of the two towns instead, leaving the A210 and A211 roads following the original route.  The nearby town of Swanley was bypassed in 1968, and the short link between the two bypasses was constructed in 1988 (to the south of the Ruxley Corner roundabout).
A section of the A20 in Ashford formed part of the Ashford By-Pass, a dual carriageway opened in 1957, which used to run from what is now the roundabout with Simone Weil Avenue to the Willesborough roundabout. Simone Weil Avenue is the original A20 bypass, but has been diverted to curtail at Canterbury Road. The eastern end of the old by-pass is now the M20.
The section between Maidstone and Ashford was the only link between the two separate sections of the M20 for 10 years during the 1980s, until the 14-mile (23 km) missing link of the motorway was completed in May 1991.
North of Folkestone the M20 becomes a dual carriageway and enters the Roundhill Tunnel before crossing over and dropping down into Dover. This part of the A20 was completed in 1993 as a project included in the Parliamentary Bill for the Channel Tunnel. During 2011, structural failures in and around the Roundhill Tunnel have caused the A20 to be closed in this area numerous times.
A20 road (England)