This is what the M2 always wanted to be, the main road from London to the Channel Ports, and one of the principal radial routes from the capital, the M20 is not just the road to Kent but also the road to Europe. So why is the important-sounding M2 still just a bypass, and the M20 the main road?
It dates back to the 1950s when direction signs pointing to the continent had a special white-on-green colour scheme (standing out from the otherwise entirely back-on-white signs). These were used for traffic heading to the Channel Ports and also Harwich in Essex. In Kent, even then, the green signs were sending port traffic down the A20, avoiding the Medway Towns.
Over time, more of the A20 was bypassed and the fragments joined to become the M20. Even so, the Department of Transport saw no reason to finish the route, and the section between Maidstone and Ashford was missing for many years.
In the early 1990s it was realised that most people coming through the Channel Tunnel would be driving off (certainly at this side, where the onward rail link has only just been finished to much fanfare). The answer was to connect up the bits and form the M20 we know and love today. The M2 was left as a short bypass while the M20 was extended and upgraded by the good fortune of being the road to Folkestone.
The M20 now runs from London all the way to Folkestone, with an extension of motorway-standard A20 to Dover. It's a very stereotypical motorway too, with three lanes all the way to Folkestone through rolling green fields and not a lot else happening. Beware the newest 1990s sections for their noisy concrete surface.