In this system, the allocation of numbers works in a hub-and-zone system, just like A- and B-roads. With London as the hub, the motorways M1 to M4 radiate outwards, with the M1 parallel to the A1 as the starting point. Where the motorway is not present, other roads fill in the gaps to form the boundaries between zones. Hence the first four routes run:
- M1 London to Leeds (then A1 as zone boundary to the Scottish border)
- M2 Medway Towns Bypass
- M3 London to Southampton
- M4 London to Pont Abraham
Unfortunately, the M2 doesn't live up to its important-sounding number, stretching only part of the way between London and Dover. It doesn't form a very effective zone boundary at all, but no matter because the boundary between zones 1 and 2 is, again, the Thames estuary and the M2 has no particular numbering significance. There is also some doubt about whether the M3 forms the zone boundary all the way to Southampton, or whether the 3/4 zone boundary takes an arbitrary line south-west from somewhere around Basingstoke.
There are two further routes, dividing what would otherwise be an extraordinarily large zone 4 into smaller pieces:
- M5 Birmingham to Exeter
- M6 Rugby to Carlisle, via Birmingham (then Solway Firth as zone boundary)
This is where you might need to take a couple of aspirin. The M5 runs south from the Midlands, but for numbering purposes we must take its start at Birmingham as being the London end of the road. It's all rather Biblical: in the beginning there was London; and London begat M1; and M1 begat M6; and M6 begat M5. The order of branching is wrong - surely M6 should start on the M5, not vice-versa - but that's the least of our worries.
The astute reader will already have seen the problem coming at us. Near Bristol, the M4 and M5 cross each other, creating what can only be described as a numbering headache. The way it works is, quite honestly, cheating: the M4 and M5 both cease to be zone boundaries after they cross. This creates some interesting situations. The western half of the M4 is in zone 5. The southern half of M5 is in zone 3. And zone 4 is a landlocked island, quite unlike any other zone. The map above right is the best way to understand it (click to see a larger version).
Just like the A- and B-road numbering system, any motorway starting within one of these zones takes the zone's number. Hence the London to Birmingham motorway is M40; the M25 proceeds clockwise around London from a start point just south of the Thames at Dartford and is therefore a zone 2 route.
The allocation of these numbers is not according to any overall plan; wherever possible they have been assigned to mimic a nearby A-road but this is not always the case and there is no requirement to do this. Hence the M56 and M23 follow their counterparts A56 and A23 closely, while the M69 and M53 actually relieve the A47 and A41 respectively. There is also no geographical or chronological system: from south to north the zone 5 routes are M50, M54, M56, M53, M57, M58, M55; the last zone 6 number to be allocated was M60. There is literally no system to determine which number a road should get within a given zone.