A major artery from London to the Kent coast, the M20 is the UK's principal connection to continental Europe for freight traffic and tourists alike.
One of two parallel motorways crossing Kent from east to west, the M20 serves Maidstone and Ashford on its way to the coast, and is the faster and more direct road to Dover. It also directly serves the Channel Tunnel and has the M26 to link it directly with the southern flank of the M25. As a result, it's the more useful road for long distance traffic, while the M2 serves more local traffic to Medway and Thanet.
The importance of this corridor, ahead of the more important-sounding A2 and M2, dates back decades. Before the Second World War, one of the UK's earliest experiments in motorway building was going to be a bypass of Maidstone. The project was halted in 1939 before work could begin, but the intention was there for a faster way to the coast using the A20. In the 1950s, when special direction signs pointing to the Channel Ports were erected, using what was then a very novel green and white colour scheme, they sent traffic down the A20, avoiding the Medway Towns.
The pre-war plans for Maidstone were dusted off and, in 1961, the A20(M) Maidstone Bypass became one of the first motorways in the UK to open. As more motorway was built, it became part of the M20, and is now the section between junctions 5 and 8. The road was extensively widened and rebuilt in the early 1990s, so today almost nothing of the 1961 motorway remains to be seen.
By the 1980s progress was stalling because the worst trouble spots had been fixed. For many years, the M20 had a gap between Maidstone and Ashford, where the old A20 was a fast enough through route. It began to look like this would always be a motorway with a hole in it.
It was the Channel Tunnel that changed everything. It was realised that most people and freight coming through the Channel Tunnel would be driving off, not continuing by rail - especially since the onward rail link on the UK side only opened years after the tunnel itself. The pressure was on to get the M20 ready for its new purpose, calling for the massive upgrade work on the Maidstone Bypass and construction of the missing link between Maidstone and Ashford.
The M20 now runs from London all the way to Folkestone, and has a de facto extension in the form of the A20 to Dover. The gentle countryside of Kent allows it to be fast and direct, with the only demanding terrain on the route lying between junctions 1 and 3 where it climbs the North Downs.
Today it's a motorway of fickle temperament - fast and free flowing most of the time, but prone to horrendous problems and even total closures when special measures are taken during disruption to Channel ferry crossings. Operation Stack was the long standing plan that saw the motorway turned into a vast lorry park, but since 2019 that's been replaced with Operation Brock, and one of the UK's longest and most elaborate contraflow systems. There's never a dull moment on the M20.