Understanding our motorway numbering arrangement really involves knowing something of how mainland Britain’s pre-motorway road numbers were allocated, so that’s our starting point: a little time here will help reduce perplexity levels later on. 

In response to numerous requests following mention of the mysterious M96 in a recent feature (well, one or two people did ask…) in this two-part series we unravel how the English, Welsh and Scottish main road and motorway network is numbered. To keep things manageable, the Northern Ireland system is not included.

This is a complex, hundred year old story, resulting in  a system which has largely stood the test of time, despite being a tortuous tale of delay and intrigue, tempered by only occasional inspiration. 

The full road numbering saga, including motorways, has enough twists and turns to make a Hollywood blockbuster plot look like a home movie: what’s here is but a brief overview, but there are references for masochists determined to know more. Who says the Western Group doesn’t give great value for money?

The origins of Britain’s road numbering system lie in the 19th century, despite which it  took officialdom until 1922 to decide on the non-motorway system still used today. 

England was divided into six clockwise-numbered zones, and Scotland into a further three. This zoning system came with plenty of tight, unbreakable rules, which have since been extensively bent, adapted, and occasionally broken or abandoned, though many rules still remain surprisingly intact over 90 years later. 

The map shows the zones, numbered clockwise around 5 historically important routes radiating out from (or “near…”) London. The A1 begins zone 1 heading to Edinburgh, for zone 2 the natural boundary of the Thames was preferred over the A2 to Dover, and the Portsmouth-bound A3 marked the start of Zone 3. Continuing clockwise, the A4 to Bath started  Zone 4, the A5 to Holyhead zone 5, and the A6 to Carlisle began zone 6. 

Next a note that nothing in life is perfect, least of all the British road numbering system. 

It incorporated various sub-rules setting arcane details which time and sanity prevent us from exploring here – and unsurprisingly in 90 years, roads have been extended, numbers changed, and liberal interpretations introduced. Thus the following is an outline, no longer holding entirely true everywhere. 

The system is founded on a “first clockwise, then radial” priority basis. So, working clockwise from any zone’s boundary road number, all other A and B routes heading outwards through the zone take that number for their first digit, with ascending numbers attached – moving clockwise then outwards across the zone. 

The west country provides plenty of “for instances.” The A3 Portsmouth road forms Zone 3 boundary. As a long distance route to Lands End, the A30 (that’s A3+0) was numbered next (thinking clockwise out of London.) 

Once all principal “outbound” roads were included, ascending numbers – always starting with the zone number – switched to cross- country routes across the zone, numbered (ideally) at increasing radial distance from London. So we get the A34 heading from Southampton to Oxford across the region, the A36 runs Southampton to Bath, the A38 Bodmin to Bristol, and the A39, the zone’s western-most route, running Newquay to Bath (think clockwise, cross country, furthest radial distance from London.) 

A tree and branch element is also apparent in places: the A3 and A30 come directly out of London, but the A31 doesn’t, so its numbered branching from A3… and the A32 branching from A31. 

These and other general rules applied nationally, with ‘A’ routes having one to four digits, and B roads a minimum 3 or 4 digits. Declining importance and increasing radial distance brought ever higher A route numbers, but B numbers were allocated only on radial distance. 

Attitudes and our road network has changed much since 1922, leading to increasing anomalies. One is the A303, once a turnpike road, now a principal route to the west country, branching off the M3 in Hampshire, later disappearing on joining the A30 in Devon. 

Following the conventions described, it could be argued this really should have been the A30 all along… but it wasn’t continuously numbered until the 1930’s, when the A30 was established, and A31 to A39 routes already existed. Renumbering the lot so soon apparently proved unattractive – so instead, A303 was plucked from London obscurity and moved further west.

One particular rigid convention stands out as a key structural component. Roads can carry the number allocated in their originating zone over a boundary… but ONLY into higher- numbered zones… unless its zone 6, where roads can enter zone 1. All clear? I thought not. 

Think clockwise, and take the A38, a pre-motorway-era long distance route starting at Bodmin in our zone 3. It enters zone 4 at Bristol, continues north through zone 5, and with extensions in recent times currently terminates in zone 6 at M1 J28 near Alfreton. This “ascending zones only” rule avoids navigational chaos resulting from roads changing number on entering each new zone. 

This system could also have worked anti-clockwise, but the suspicion is that in a 1920’s non-digital age, clockwise thinking was more naturally intuitive for rule-generating minions. Anti-clockwise zones would of course have rendered an alternative road numbering landscape utterly alien today. On such things does life’s comfortably settled motoring existence depend. 

The Scottish system follows similar rules, with zone boundaries radiating from Edinburgh. Zone 7 heads north from the southbound A7 Carlisle road, Zone 8 covers Glasgow to the Western Isles, while Zone 9 looks towards John o’ Groats and the Shetlands. 

There’s much, much more, but here ends our primer on the British road numbering system. Part two digs deep to uncover and open the can of worms revealing how our motorways were numbered.


A list of road numbers that have either never been allocated or are not currently in use is here:http://www.sabre-roads.org.uk/wiki/index.php?title=Unallocated_numbers 

A list of some anomalies in the A and B road numbering system is here:http://www.sabre-roads.org.uk/wiki/index.php?title=Numbering_anomalies

Books and publicationsRees Jeffreys, The King’s Highway. London, The Batchworth Press, 1949.

Dave Moss
Dave Moss

Dave Moss has a lifetime connection with the world of motoring. His father was a time-served skilled engineer from an age when car repairs really meant repairs: he ran his own garage from the 1930’s to the 60’s, while Mum was the boss’s secretary at a big Austin distributor. Both worked their entire lives in the motor trade, so if motor oil’s not in Dave’s blood, its surely a very close thing.

Though qualified in Electronics, for Dave it seemed a natural step into restoring a succession of classic cars, culminating in a variety of Minis. Writing and broadcasting about these, and a widening range of motoring matters ancient and modern, gathered pace in the 1970’s and has taken over since. Topics nowadays range across the modern motoring mainstream to the offbeat and more arcane aspects of motoring history, and outlets embrace books, websites national and international magazines, newspapers, radio programmes, phone-ins and guest appearances. Spare time: hard graft on the garage floor attending to vehicles old and new. Latest projects: that 1968 Mini Cooper S has finally moved again after 30 years, and when the paint is finished, the 1960 Morris Mini 850 will also soon be ready for the road again…

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