Major roads have markerposts that start at one end of the road and count the kilometres to the other. Except that, on the UK road network, things are never that simple.
Here’s a word of warning: some of our features are more nerdy than others. Sometimes we just enjoy a drive on the A39, or look at some nice old fingerposts, or explain signs that seem to confuse lots of drivers. Other times we go a bit deeper and talk about the history of pedestrian crossings or an obscure motorway in Lancashire.
Once in a while, though, we really plumb the depths of road geekery. This is one of those times. If you enjoyed reading about the number codes for electronic signs on motorways, then this one is for you. (And if not… then here’s a story about a cake.)
Highway authorities use markerpost numbers to identify the location of roadside objects and problems that need fixing. You can direct your maintenance crew to a pothole or a damaged crash barrier without any fuss if you tell them it’s located at, say, markerpost 101.5. It’s an address, ticking off the metres and the kilometres of a road like the markings on a tape measure. You'll see them as small grey numbers down the left side of our Motorway Database exit lists.
You’d think - if you were new to the UK’s road network, perhaps, and still optimistically believed that the roads were organised, and sensible, and governed by logic - you’d think that markerposts would begin at zero and count up until they reached the end. Thoughts of that kind may be abandoned now.
Something you can count on
For a system of markerposts, you choose a starting point and then you begin counting. One end of the road is chosen as the start: this is called the “datum” or “origin”. It may coincide with a meaningful landmark or location.
Then, you measure from that point along the centre line of your road, and place markers by the roadside with the distance marked on them. In the UK, we use white plastic posts with a band of green or blue at the top, and we put them at 100 metre intervals. Typically they will bear two numbers, kilometres at the top and tenths below. On motorways they will also have a symbol pointing to the nearest emergency telephone. In England they also appear on Driver Location Signs.
On anything bigger than a single carriageway road, you might supplement this system with carriageway letters, to identify an individual strip of tarmac. Typically, A and B identify the main carriageways.
- A will travel Away from the origin, with numbers increasing, and
- B will travel Back towards it, with numbers counting down.
Junction numbers, where they exist, will count upwards away from the origin.
Official guidance says that C and D identify auxiliary carriageways, like on the M20 near Maidstone. By happy coincidence these parallel carriageways are often called “CD lanes”, short for “collector-distributor”. In practice the parallel carriageways on the M20 are actually called L and K, and we've been unable to find a real life example of C and D in use.
J, K, L and M identify sliproads at junctions. These will share numbers with the A and B carriageways, so they need different letters to avoid duplication.
Most other letters are rarely, if ever, used, and the letters I and O are avoided because they look too much like numbers. The obvious question is what happened to E, F, G and H, but there seems to be no particular answer. This is just the long-standing convention.
Having established these very straightforward concepts, we should be able to visit the first junction on any motorway or major A-road and find its markerpost zero. Right?
Wrong. If you start at zero, you leave no scope for any future extension backwards from the origin. For that reason, we are about to find a lot of roads that don’t have a zero.
First London, then the world
For many years, distances to London have been measured from the statue of Charles I in Trafalgar Square, which stands on the site of the original Charing Cross. When motorways began spreading outwards from the capital, their markerposts counted from there, neatly allowing for any inward extension of the motorway towards the centre.
As a result, you’ll find the M1 begins counting at markerpost 11.0. It’s actually 10km from the statue as the crow flies, but was presumably expected to be 11km away by road. For the same reason, you’ll find the M11 starts counting at 11.7, the M4 at 12.2, and the M3 begins at 24.3. They are all counting from Charing Cross.
Some roads further away do the same, despite their non-existent relationship to the statue. The M6, for example, branches off the M1 in Leicestershire, and continues the same numbering, so it begins at 132.0. The M18 does the same, counting up from an inherited origin of 250.2 in South Yorkshire.
The M40 starts at Denham, on the fringe of Greater London, at marker post 29.6. This sounds about right until you try counting back towards the statue. Whichever route you choose it’s at least a mile too high to be counting from Charing Cross. Interestingly, though, the first 400m of the M40 are on top of what used to be the end of the A40 Western Avenue, and the 30km mark is precisely where the motorway branches off the existing road. 30km might just be an arbitrary number above zero - in which case the M40 has a “false origin”.
False origins and false hopes
Roads that start counting from a base above zero have a “false origin” - which is to say, one that marks no particular place, unlike the statue of Charles I.
Often this is in case the road is ever extended backwards from its start point: depending on the road, there might be a very precise allowance, for an extension that was planned in detail, or a big round number that covers all eventualities. The M40 is probably in the latter group, with 30km allowing more eastward extension than could ever be built, guaranteeing the motorway would never run out of numbers.
Once you start looking, you realise there are a lot of false origins.
The M69, running between Coventry and Leicester, originally started at markerpost 100, since there were all sorts of ideas to extend it south and no clear picture of where it would go or how long it would be. When the Coventry Eastern Bypass opened, some of that slack was used up, though not much - it now begins at marker 99.7.
In Dorset, the A35 between Bere Regis and Dorchester was upgraded in the 1990s, and the new road begins at a clean 100km. This was done to ensure completely different numbers to those used on the A31 just to the east.
Others are harder to explain. The M27's first marker is 1.6, but it’s unclear why the zero point might be a mile west of the junction. Similarly the little M181 in Lincolnshire starts at a weirdly specific 8.8, even though there has never been a plan for a southward extension and 8.8km gets you nowhere useful anyway.
The M6 Toll branches off the M6, but doesn’t inherit the Charing Cross-based numbers, and instead starts counting at 9.5. That puts its origin around Corley Services, for reasons unknown.
Possibly the best and most unarguable false origin, though, belongs to the M55. It runs from Preston to Blackpool, and since the motorway could theoretically extend any distance east, but could go no further west than the sea, its zero point is on the promenade at Blackpool. It's possibly the only motorway where the junction numbers count in the opposite direction to the marker posts.
Something borrowed, something blue
Often a smaller route will inherit its origin from its parent, counting from the point where the two meet.
The M271 in Southampton does this, meeting the M27 at marker 10.6. The trouble is that the M271 runs both north and south from that junction, and the numbers count upwards both ways. So it has two 10.7s, two 10.8s, and so on. To avoid duplication, there's no A or B carriageway; instead, north of the M27 are carriageways J and K, while south of it are L and M.
Better yet, the M271 has a junction of its own at Nursling, whose sliproads are weirdly carriageways N, P, R and S. As a result, markerposts 11.7 and 11.8 occur eight times across the M27 and M271, differentiated by carriageway letters A, B, J, K, L, M, P and R.
Equally confused is the M26, linking the M25 at Sevenoaks to the M20 near Wrotham. It has its own numbers, beginning at zero where it branches off the M25, but inherits carriageway letters from the M20, considering itself a pair of long sliproads lettered L and K.
Double trouble
Real trouble begins where a single road has duplicate numbers or jumbled letters.
The A74(M) continues from the M6, so its origin is in London. But that persists only to J16, where the direction suddenly reverses. This is because the M74 started construction from the other end at a much earlier date, so its origin is Glasgow. Luckily there are no duplicates: the highest marker on the Glasgow-based system is lower than the first marker on the London-based system.
The M74’s origin was fixed in the expectation that it would eventually reach an unbuilt section of Inner Ring Road at Glasgow Green. Its eventual extension took it much further west, meaning it ran out of numbers. It may be the only motorway whose false origin failed to provide enough headroom, and its zero point falls short of its terminus on the M8. As a result, west of the 0 marker post are a further set numbered Z0 - zero zero, if you like - to fill the gap.
The M23 and A23 swap directions too. The M23 has its origin at Charing Cross, so it counts southward from 27.6km at Hooley. But the A23's origin is the A27 near Brighton and it counts north from there. The two systems meet at Crawley, but again - because the M23 begins so far above zero - there are no duplicates. The A23’s A and B carriageways refuse to join in with this madness and instead match the M23, so south of Crawley the B counts up and the A counts down.
Not so lucky is the A3, whose Surrey section counts southwards from a zero marker at the Greater London boundary near Kingston, and whose Hampshire section counts northwards from the A27. This generates overlapping numbers and conflicting A/B carriageways. Worse, many of the A3’s bridges are numbered not in reference to its markerposts but instead counting in kilometres from the City of London, introducing a third set of overlapping numbers.
The M56 splits into two branches at junction 3, one short stub joining Princess Parkway towards Manchester, and the other forming the Sharston Spur towards Stockport. Both call themselves the A and B carriageways, so markers 9.6 to 10.0 are unhelpfully duplicated. Its lowest marker is 6.5, at Kingsway Interchange on the Sharston Spur, but the origin appears to be somewhere in Manchester City Centre via the other branch.
Patient zero
The award for messiest numbering and most duplicates, though, goes to the A1. It has at least four origin points. Its first is, naturally, London - probably at St Pauls, where the road starts, though that’s not entirely clear. This scheme reaches 99.9km at the A14 near Huntingdon and then, for no reason anyone can adequately explain, it resets to zero.
From Huntingdon the numbers count up again, giving the second hundred kilometres of the route the same numbers as the first. Then, at the start of the A1(M) Doncaster Bypass, Blyth Interchange is a third zero point. From there, to maximise confusion, sections of A1(M) follow the third origin at Blyth, while sections numbered A1 follow the second origin at Huntingdon. The motorway is now nearly continuous, but until a few years ago the markers could be seen to switch between the two systems all the way through Yorkshire.
At J56 near Barton, where the Durham section of motorway begins, the numbers reset back to zero for a fourth time, because why not?
A further section of A1 expressway then exists in Scotland, between Edinburgh and Dunbar, but it appears to have no markerposts at all, which is probably wise.
Does any of this matter? Probably not. The numbers we have, however bizarre they may be, evidently don't prevent maintenance work being done and stranded drivers being found. They may be messy but they serve their purpose. But it is remarkable to discover that something which gives every impression of efficiency is actually very haphazard. The more you learn about the UK's road network, the more familiar that feeling becomes.
Comments
There’s a theory (hat tip to the hive mind if SABRE) that it’s because of the duplicate marker post numbers elsewhere on the A1(M). DLS must be totally unique in the whole country, so there’s no scope for two markers with the same number on the same road. The D (for Durham?) and the slash make them sufficiently different.
I don't think the D is for Durham; the motorway was originally the Darlington Motorway at this point, with the Durham Motorway coming a few years later. My thoughts have always been that it was:
A from M25 to Junction 10
B for the Doncaster Bypass
C for the North Yorkshire (Darrington to Dishforth)
D for the Barton - Birtley section
These were the sections in place when these blue marker plates came into use - since then further parts of the A1(M) have been created, e.g. Alconbury - Peterborough, the M1-A1 link, the Dishforth to Barton section.
The start point for the D section is clearly Barton. Latest Google pictures show 139.1 as part of northbound A1(M) and then a 0.6 (0/6) shortly after on the original D section
However, the A, B and C sections don't appear to have any marker posts to back up this point - neither do the A194(M) or A195(M) whilst the A66(M) does, despite all three being effectively spurs of the A1(M). Does that add credence to the A195(M) being a separate motorway rather than a spur?
Agree with your point, though that each element of the A1(M) needed its own reference (e.g. D) to avoid duplication.
This is a nice idea but I don’t think this is the reason for the letter D. Driver Location Signs were first trialled around 2004/2005, at which time there were six separate lengths of A1(M), and they very definitely post-date the opening of the Peterborough and Aberford sections which were completed in the late 1990s. So sequential lettering of that type would make the Darlington/Durham section F, not D.
The A1(M) has only patchy DLS coverage, but most of the run from Darrington to Barton has them, albeit with a mix of either “A1M” or “A1(M)” on the top line depending who erected them - the DBFO operator on the Aberford Bypass seems to avoid brackets. They don’t use any letter to differentiate themselves (they would have a C on your system) and they follow the Blyth origin.
Just to throw another spanner in the works, the A1(M) around Doncaster still has old-fashioned chainage markers instead of DLS - instead of metric units, it shows the distance in miles and chains. I believe the origin point is at the southern end of the bypass.
As Bryn mentions, the DLS on the Durham section are indeed all covered over. When originally put in place, they displayed the distance from London. I once broke down in a coach on this section and used the opportunity to photograph one, with the original distance clearly visible under the patch. It's somewhere buried on a page on Sabre
Another motorway besides the M55 where the junction numbers run in the opposite direction to the marker posts is the A3(M). The junction numbers start at 1 where the motorway diverges from the original (and still current) A3 but, as mentioned in the section "Double Trouble", the origin for the marker posts is at the other end on the A27.
Im scratching my head on this. The reason being that the use of metric for speed limits signs and for distances to locations on road signs is illegal. Likewise when you take your car for an MOT you cant have a speedometer with kilometres only, it must have both. And correct me if Im wrong the speedo must have miles on the outside.
And not long before he finished as PM Tony Blair was asked at PMQs about metric road signs and he said that it would cost at least 700 million quid to change them, and he wasnt in favour of doing it.
The thing that makes metric speed limits, regulatory signs and distances illegal is TSRGD, the document that sets out standards for road signs. It’s not a blanket law that says “no metric signs”, it’s just that all the signs intended for motorists are specified as imperial. There are exceptions to that anyway: bridge heights, for example, have to be in both metric and imperial.
TSRGD specifies markerposts too, but doesn’t say how the numbers on them should be chosen or allocated. Given that roads are designed, built and maintained in metric, and have been for half a century, it makes sense that engineers working entirely in metric use metres and kilometres to specify locations on those roads.
To put it another way - the idea that the UK’s roads are imperial is really window dressing for road users. Look under the bonnet and it’s metric all the way down.
I should have added that my background is aviation. I recall the incident in Canada when the Boeing 767 took off with only half the fuel it should have had. I wont go in the full facts but it was a mistake involving Imperial and metric.
Most distances shown on signs - in the TSRGD and generally in the design stage - have a 10% leeway to account for the difference between yards and metres. The giveaway is when you see sub-plates showing "440 yards", where a designer really means 400m and hasn't remembered that they can sort of mean one and the same thing.
Plus marker posts are never exactly 100m apart and should never be assumed as such.
If they weren't designed using "m" for miles rather than metres, conversion could have been made entirely unnecessary. But we are where we are.
Everything on the road network is specified in metric - road design, signage etc and has been for fifty odd years. The only exceptions are distances on signs which are always converted from metric plans (hence why the distances are invariably round metric distances, 220 yards - 200 metres etc); and speed limits where the unit is almost irrelevant, which is why people refer to a "30 limit", "20's plenty" etc.
Speedometers are not checked in the UK MoT - they do not have to be working, you could cover it with duct tape if you wished. My car (Japanese import) only reads in km/h and it's never a problem. It's just worth reminding the MoT tester the odometer is in kilometres or else it will be recorded wrongly.
Im surprised to learn that about speedometers. Bloke I worked with a few years ago had brought a car over from France. He took it for an MOT and was told he had to have a speedo in Imperial. And he contacted VOSA and was told that was correct.
I think the reason for the odd distances is more likely that ½ mile is 880 yd, ¼ mile is 440 yd and ⅛ mile is 220 yd, or a furlong. They predate properly designed roads.
Similarly, my first motorbike was a 125 cc Yamaha imported from Austria or Switzerland, which had a speedo that only displayed km/h, and it had consistently passed MOTs for 13 years before I got it. (It had German-language manufacturer's stickers on it, but no 50 mph limiter, so it hadn't come from Germany itself.)
Confusingly, the actual speeds and distances it read were in mph thanks to a device fitted by a previous owner. I discovered this when my confusion and annoyance got the best of me, and I bought a second hand dual-unit display and switched it out for the old one, determined to resolve the matter once and for all, only for the distances recorded on the odometer to come out at 5/8 of their expected value; turned out that little brass cylinder at the wheel end of the speedo cable didn't match any of the components in the shop manual, and was an aftermarket km-to-miles reducer gear.
"Likewise when you take your car for an MOT you cant have a speedometer with kilometres only, it must have both. And correct me if Im wrong the speedo must have miles on the outside."
Not quite. Ex-MOD vehicles have metric speedos with small MPH figures on the inner ring. Been getting my pair of old military Land Rovers with this format of speedo MOTd since 1988.
Wild speculation with little evidence: Corley Services are approximately where one starts seeing those "M6 Toll Traffic Information" matrix displays on the M6. Those actually have M6 addresses, but is it possible they were intending to give them M6 Toll addresses and set the datum far enough back to accommodate that?
Good thought. It may be even simpler than that - if they (and other changes on the M6 leading up to the start of the new motorway) were installed as part of the M6 Toll contract, then the contract may have covered the length of M6 right back to Corley, in which case there may have been a need to reference those locations in construction drawings etc.
The M32 is another motorway where the marker posts go in the wrong direction. As far as I can tell, distances are counting up from either the Bearpit or Old Market interchange, both of which are slightly beyond the intended terminus at Cabot Circus.
I've had a look there is one example of C and D being used as intended – well half an example because it only has a D and no C. It's on the section of M55 between the M6 and junction 1, and it has just the one sign confirming it here.
You'll also find a C and a D at the M4/M5 junction, which uses CDEF for the left turns and GHJK for the right turns. Meanwhile the junction 18/18a complex to the south uses all the letters S to Z.
A question: how are loop roads measured? For example, where is "Mile Zero" (or "Kilometer Zero") on the M25 orbital road?
There's no particular rule. In the case of the M25, there is no zero marker post; the lowest is 2.1km just north of the Dartford Crossing and the numbers count clockwise until they reach 187.3km. Of course 187.3 is then followed, a hundred metres later, by 2.1km! If you count backwards, zero would be around the bridge carrying the north side of the roundabout over the M25 at junction 30. I don't know why that would be chosen as the origin.
The M60 is much simpler - it gained its current number in 2000, and its junction numbers and markerposts were all re-worked at that time. The M60's markerpost 0 is at junction 1 in Stockport, and the numbers count around the circle to 56.7km.
I happened to drive this exact area the other day, and the signage in the area is very peculiar.
Travelling clockwise, the actual M25 "end of restrictions" sign it located just after the J30/A13 off-slip, which seems like an odd place since the road doesn't actually have to revert to all-purpose until the following on-slip - in other words, there's technically a small stretch of A282 that you can only reach by motorway. However, the driver location signs continue to say M25 and count as if they are M25 until just after the crossing control building where it abruptly resets to A282 2.1km, as you say.
My theory is that the 'zero point' is where the M25's restrictions end, as that tallies with what you said about it being the north side of the roundabout, which would make some sense. The thing that doesn't make sense is why the signs themselves continue to claim the road is M25 when it most definitely is not - not only has the end of restrictions sign already passed, but the subsequent on-slips all confirm the road is A282.
Slightly off the main point of the article but the M60 junction numbers were apportioned so that the majority of the former M62 motorway junction numbers were unchanged when the motorway was renumbered.
Re: locations of C carriageways. When Operation Brock is active on the M20 then the contraflow lane is officially designated the C carriageway.
Do you mean are the rare occasions Brock isn't operative the C carriageway is closed?
Just to be different, the M26 has K & L Marker Signs: K counting up from the Chevening Interchange, and L counting down towards the Chevening Interchange.
The A90 AWPR northbound has marker posts going from "00 / 0N" to "38 / 8N"; southbound is simply "38 / 8S to 00 / 0S". The A956, more curiously, has marker posts going from "56 / 5E" to "50 / 0E" eastbound (and "50 / 0W" to "56 / 5W" westbound). Maybe chosen to avoid duplication with the A90 numbers, as there is only another ~12km of (non-Special) A956 left going eastbound!
I recently drove along the sliproads taking you from the M20 Westbound to the M25 Northbound and then subsequently making a return journey M25 Southbound to M20 Eastbound, when I notices some very odd numbering of the Driver Location Signs, when it read:
West-North
M20
L
MD.10
South-East:
M20
J
DM.12
These signs are visible on google maps (Note: The sliproads I am referring to are the ones that completely bypass M25 Junction 3 but the ones that go directly onto the carriageway)
That is indeed very strange. The West-North slip shares the same carriageway letter (L) as the slip-road it is an offshoot of, so I guess there is some logic there, but it would have made more sense to simply give it its own carriageway letter. Even more-so with the South-East (J), as that slip doesn't actually connect to any of the other existing slips.
Perhaps 'D' standards for Dartford and 'M' Maidstone? So the South-East slip is DM, or "Dartford To Maidstone", and West-North is vice-versa.
Out of interest, what happens where the M90 splits, with one branch going up to the Broxden roundabout and the other across the Friarton bridge? I assume the numbering can't be duplicated? And what about the Queensferry Crossing and the new stretch round to meet up with the old road, given that the Forth Road Bridge is still open?
I did a Google Maps Streetview Safari and my conclusions are:
- There don't appear to be any DLS signs on the M90 at all.
- I can't zoom in enough to read the marker posts.
- Based on the SOS phonebox numbers, it appears that the carriageways are numbered 'N' for north-bound and 'S' for south-bound. The SOS boxes on the Broxden roundabout spur are numbered in the 6xx range, the Friarton bridge ones in the 8xx range.
- I discovered that the M90 is actually split into two at the very beginning - part of what really ought to be M90 is actually A90 for some reason, between junctions 1 and 1A. Madness.
- Based on the SOS phonebox numbers, it appears that the M90/A90/M90-again complex south of the Queensferry bridge are numbered consecutively as-if the A90 section were still part of the M90 (which, honestly, it practically might as well be: aside from the green signage, the standard of the road is indistinguishable from the M90 at either end!)
The M90 won’t have any DLS - they are exclusively an English thing.
The current works to upgrade the M6 between J21A-26 to smart motorway has seen all the verge side road signs removed - the temporary DLS erected on the steel site barriers are quite a clever design - they're stacked vertically almost as if they were lifted straight from the MUTCD over in the USA.
https://goo.gl/maps/jCg4VZxptmBof9GLA here's one in action.
The southbound M74 marker posts from the M8 to the "origin" are X0, I guess Z0 applies on the northbound side but I haven't been able to check this!
Another peculiarity I noticed just the other day: On the westbound off-slip of M25 J8, the DLS signs cheerfully announce that you're on the 'A' carriageway.... but you're not, you're on the 'J' slip, as the SOS phone numbering clearly shows.
Really there ought to be a pair of DLS signs, one for the main carriageway and one for the slip, showing 'A' and 'J' respectively - which is something I have seen done in a few other places.
In most cases this would be a matter of pedantry, and unlikely to cause any real issues, but as this particular slip road is a mile or two long, I think it could genuinely lead to confusion - if you break down on the slip and report to the AA that you're next to the "M25 A 50.3" sign... if no-one enquires further, that AA van is going to drive straight past you on the main carriageway! Even if they do spot you, they'll have to drive all the way to J9 to turn around, then back to J6 to turn around again... a significant delay!
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Sources
- Standards for Highways: CD 193 Driver Location Signs [PDF]
Picture credits
- Photograph of Corley Services is taken from an original by A J Paxton and used under this Creative Commons licence.
- Photograph of M271 is taken from an original by Lewis Clarke and used under this Creative Commons licence.
- Aerial photograph of A3 near Liphook is taken from an original by Chris and used under this Creative Commons licence.
The Durham section of the A1(M) seems to enjoy different notation too - see here: https://goo.gl/maps/xmk62xmRete7hSX39
14/2 D - and it appears to be a coverplate.