The start of work on the motorway network in the late 1950s created a whole new type of road and called for a whole new set of numbers. Despite initial plans to number motorways as part of the A-road network (where, for example, the A50 was proposed for what is now the M1), it didn't take long for the idea of separate numbers with the prefix "M" for motorway to catch on.
This page looks at how those numbers are allocated across Great Britain, and also how those pesky A(M) numbers work.
England and Wales
Unlike the unified and elementary system used for numbering all-purpose roads, the system for motorways is fragmented and, in places, has the potential to give you a headache if you think about it for too long.
The first surprise is that it's actually two systems. By the late 1950s, Scotland was looking after its own trunk roads, thank you very much, and so we will turn to Scottish motorways in a moment. The more elaborate system is the one shared by England and Wales.
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.
The twisted logic of motorway zones in England and Wales is bad enough as it is. If you've managed to get this far without breaking into a sweat, you might like to tackle this question.
In the 1960s and 1970s, a motorway route was proposed across Worcestershire and Warwickshire, connecting the M50 at Strensham to the M69 north of Coventry (see left). Once these routes were connected it's a fairly safe bet that they would have become a continuous route and the whole thing would have been given one number.
The question is - given that motorways are expected to travel clockwise around the hub-and-spoke system, which zone would this motorway take its number from? (The answer is at the bottom if you need it - no peeking please.)
So from chaos to simplicity: the Scottish motorway numbering system is a piece of cake. It can be explained in three steps, including the whole process of selecting a route and constructing the road, which is quite impressive. So, if you're ready, this is all you need to do:
- Identify a congested route that should be relieved by the construction of a new motorway.
- Build a motorway next to it.
- Use the existing road's number for the motorway.
Yes, honestly, it's as simple as that. It almost makes up for the England and Wales system. M8 replaces A8; M9 replaces (the southern part of) A9; M74 replaces A74; M80 replaces A80, twice. It solves the often-asked question about why there is no M7: it's because there has never been a need to replace the A7 with a motorway.
There are two little dark clouds on our otherwise clear blue horizon. The first is the consideration that any motorway that replaces a short A-road gets a ridiculous number: M876 is a vital link in Scotland's motorway network, but it parallels the A876 and therefore it gets an outrageous number even though all the numbers M81 to M89 are unassigned.
Slightly more problematic is what happens if you build a motorway on a whole new line, not parallel to an A-road. Scotland never built motorways in this way, like England's M1 or M6, but there is one that escapes. The M73 takes its number from an A-road that lies some way away and which performs a completely different function. But it's, you know, kind of parallel. So that's fine.
Ax(M): the other kind of motorway
There is another type of motorway number and it's quite unlike anything used anywhere else in the world. It goes by the less-than-snappy name of "A-road with motorway restrictions". These roads are full-blown motorways - the restrictions are the same and the standard of engineering matches the best and worst of regular motorways. They just don't have a motorway number.
Initially they were intended as short motorway bypasses of sections of A-road, and when they were connected by other lengths of motorway, they were to assume a full M-number. So A20(M) ate its vegetables, grew up to be big and strong and now goes by the name M20. The idea was stretched to include spurs of motorways where the spur and the motorway together form a bypass of a section of A-road. The spur then takes the cumbersome number; hence the A48(M) and its parent motorway M4 bypass a section of A48 between Cardiff and Newport.
Usually the corresponding A-road will vanish where such a road has been created. It's meant to, anyway: the opening of the A20(M) Maidstone Bypass caused the old A20 through the town to become A2020. When the Bypass became part of M20 the old road reverted to A20 again. But there's always a straggler or two; the A3(M) never shook off the A3 which parallels it all the way through Waterlooville, for example.
Ax(M) designations remain the most unusual and awkward road numbers in the world. This is never less obvious than with the case of the Great North Road. It has been bypassed many times by various isolated sections of motorway, but because the number M1 has long since been taken, its bypasses - even when connected to form lengths of thirty miles or more - will forever rejoice in the instantly-forgettable name A1(M).
The Midland motorway conundrum
The new motorway between M50 and M69 would have to have a number beginning with a 4, despite poking out of zone 4 at both ends. The reason is simple: forget the 'clockwise' rule for a moment and instead think of it this way. A motorway can enter zones with a number higher than its first digit but not lower. So no zone 5 or zone 6 motorway can enter zone 4, but a zone 4 motorway can enter both zones 5 and 6.
No number was ever assigned to the proposal, but M46 would probably be most appropriate.