The main junctions on the London to M25 section are: The A20 then passes the racing circuit at Brands Hatch before descending steeply from the North Downs escarpment past Wrotham then West Malling and on to the county town of Maidstone.

It takes a sharp turn left at Newingreen (the site of the UK's first motel) before entering Folkestone via Cheriton, passing the vehicular entrance to the Channel Tunnel, forming part of the town's original bypass as the trunk road from the M20.

The route then follows the coastline, tunnelling through the hills and descending to the docks in Dover, where it meets the A2 again coming down from Canterbury.

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.

[6] North of Folkestone the M20 becomes a dual carriageway and enters the Roundhill Tunnel before crossing over and dropping down into Dover.