Procedure
The M20 is the main route from London and most of Britain to the Kent ports and Channel Tunnel. It runs south-east from the M25 motorway, carrying traffic to and from the rest of the motorway network. As Britain is an island, large volumes of goods traffic move between the UK and continental Europe via Kent.
Lorries are parked on the coast-bound carriageway, with the road closed to all other traffic. Phase 1 involves closing the road between Junctions 11 (Hythe) and 12 (Cheriton), with the A20 providing an alternative route. Phase 2 involves closing the M20 between Maidstone and Ashford (Junctions 8 to 9) with the A20 being used as the diversionary road again. Phase 3 involves closing the M20 London bound between junctions 9 and 8 (Ashford to Maidstone).
This stacking can close the motorway for days at a time, and as a result, Operation Stack has been controversial.
Read more about this topic: Operation Stack