Matrix signs reloaded

Published: 28 July 2017

England's motorways are increasingly loaded with new technology - especially those that have become Smart Motorways. All those signals, cameras and other gadgets have to be identified somehow, and it turns out there's an ingenious system designed to do just that.

Road enthusiasts love a system, of course. Discussions about the intricacies and inconsistencies of road numbering are a perennial favourite. So it will be exciting news to anyone who likes to spot a pattern and collect a full set that there's another numbering system afoot on the motorway network - one right in front of you that's almost impossible to decipher unless it's been explained to you.

A silly mistake

An MS1 signal, accurately drawn but inaccurately numbered
An MS1 signal, accurately drawn but inaccurately numbered

It all started when we published an article called Mixed Signals last year, which looked in detail at the development of electronic signs through the 1960s, culminating in the Motorway Signal (or MS1). The article included an animation showing an MS1 signal in operation, cycling through some of its messages and showing off its exciting amber lights.

There's nothing wrong with the signal animation, but it turns out there was something wrong with the drawing. We were contacted by a nice man called John who has worked on motorway signalling for the last twenty years and these days works on Smart Motorway schemes. He pointed out that we'd given our MS1 a markerpost number - 117/5 - not a signal number. You can tell straight away because it's not formatted correctly, and most importantly it hasn't got a road code.

So, of course, we asked the question anyone would ask.

What's a road code?

The short answer is that it's used to make a unique address for a roadside asset. The long answer is... longer.

How to properly address a signal

We'll get to road codes and numbers again in a moment, but first we should talk about the Highways Agency Traffic Management System, or HATMS, which is used by seven Regional Control Centres. The system they use to run the technology on motorways has a number of sub-systems. Each is given a three-letter code. Here are some of them:

  • SIG for SIGnals, which operates single-symbol panels like the MS1 above
  • MSS for the Message Sign Subsystem, which runs bigger signals that carry several lines of text
  • MID for Motorway Incident Detection and Automatic Signalling, or MIDAS, which detects traffic flow and can set signals automatically in some places
  • MET for METeorological equipment: the small weather stations at the roadside that detect wind, fog and so on
  • HSM for Hard Shoulder Management systems
  • TID for the TIDal flow system on the A38(M) in Birmingham

Different types of equipment connected to the HATMS system are therefore identified by having different letter codes, and two items of equipment at the same location are easily distinguished. (They are not normally written on the equipment for the obvious reason that when you're standing in front of it you can tell a weather station from a motorway signal, and if you can't you probably shouldn't be sent out to fix them.) Some equipment contains more than one type of signal - for example, an MS3 signal actually contains both SIG and MSS equipment, which can be addressed separately.

An MS3 signal, with three lines of text (MSS) and a signal panel (SIG). Click to enlarge

An MS3 signal, with three lines of text (MSS) and a signal panel (SIG). Click to enlarge

The location itself is based on the markerpost number, but it can't be just the markerpost number because lots of motorways start at zero and count up, so the M1 has a markerpost 53.3, and so does the M2, and the M3, and the M4... so to avoid duplicating addresses, and confusing one place with another, the markerposts have to be converted to something unique. That's where the road code comes in.

The road code is a four-digit number that is added to the full markerpost number to generate a four-digit address. The address is then unique to the system managing it. There can be duplication of addresses across the whole of the UK, but there is never duplication in the addresses managed by each Regional Control Centre.

Some roads travel through several Regional Control Centres, and some have markerposts beginning well above zero, so devising a system like this must have been a complete headache, but the road codes are cleverly arranged in such a way that no address is duplicated within a single RCC's area. 

This is the full list of road codes, which includes at least one strange historical relic. 

Route Road code Route Road code Route Road code Route Road code
M1 2000 M42 6100 M66 4000 A3(M) 5000
M2 8000 M45 5000 M67 8000 A11 6700
M3 1000 M48 2000 M69 8000 A14(M) 7000
M4 2000 M49 6000 M180 1000 A14 1000
M5 7000 M50 2000 M181 1900 A20(M) 7000
M6 4000 M53 9000 M275 9000 A38(M) 3800 & 3000
M11 6000 M54 3100 M600* 1000 A45 7000
M18 7000 M55 5000 M602 6000 A46 4600 (East Mids)
M20 6000 M56 8000 M606 2000 A46 8000 (West Mids)
M23 3000 M57 2000 M621 1000 A66(M) 4000
M25 4000 M58 7000 A1(M) 9000 (NE) A180 1000
M26 2000 M60 9000 A1(M) 7000 (NE) A194(M) 9000
M27 9000 M61 2000 A1(M) 2000 (J14-17) A329(M) 9000
M32 5000 M62 1000 A1(M) 9000 (J1-11) A404(M) 2000
M40 8000 M65 4000 A2 8000 A627(M) 6000

* There is no M600 - it's actually referring to the M6 Toll. Some Highways England computer systems don't allow for a road number as wilfully obtuse as "M6 Toll", so the dummy number M600 is used in its place.

To get a unique address for a signal, you add the road code to the full markerpost number. So if our animated MS1 signal, which appears to be at markerpost 117/5, happened to be on the A carriageway of the M5, the number on the front would be 8175 (7000 + 1175), and its full address would be SIG M5/8175A.

The final element of the system optionally allows for multiple instances of the same type of equipment sharing the same location by adding a number to the end of their address. So, for example, an overhead gantry on the M42 with three signals mounted over three traffic lanes might carry SIG M42/6212A1, SIG M42/6212A2 and SIG M42/6212A3.

We can even try it out for real if you like.

An MS4 signal on the M25 between junctions 24 and 25. Click to enlarge

An MS4 signal on the M25 between junctions 24 and 25. Click to enlarge

This MS4 signal was installed on the M25 between junctions 24 and 25 when it was upgraded to Smart Motorway a few years ago. The number printed on its support should be the markerpost number added to the M25's road code - which is 4000.

Let's see.

The address of the MS4 with a nearby markerpost number

The address of the MS4 with a nearby markerpost number

It's positioned roughly 100 metres before the Driver Location Sign at markerpost 145.3, so the signal itself is probably closer to 145.2. 1452 + 4000 = 5452, and sure enough that's the number on the sign.

Perfectly imperfect

It may be that it's enough for you to have learned that this is how the system works. (It may equally be that you stopped reading a long time ago, in which case don't worry, we'll talk about something else next time.) But if you're the sort of person who enjoys learning about a clever system like this, you will almost certainly enjoy it even more if you know it's not quite right.

The A1, you see, duplicates its markerpost numbers, and it's a particular problem in the North East. The Driver Location Signs on the A1(M) in County Durham have had to be patched with new numbers already - you can see them as you drive past - and two road codes have had to be assigned to make sure it's still possible to uniquely identify roadside equipment reporting to the North East Regional Control Centre. Some have a code of 7000 and others 9000.

So, next time you're on the motorway and want something to occupy your mind, take a look at the signals and telephones at the roadside and see if you can work out the road code. And if that bit of mental arithmetic is too simple to occupy you for long, go for a ride on the A1(M) around Darlington and see if you can work out how to fix the markerposts. It's been a problem for years and there are engineers in a control room nearby who will thank you.

We'd love to hear about other interesting systems and conventions like this - if you know of other similar topics we should cover, leave us a message in the comments section below.


What about lighting columns? These have nice long numbers on them, too. Do they follow the same system?

This is a very good question and the answer is that I don't know! Does anyone else?

Sven 29 July 2017

Slight mistake? Sign in photo is 5452. Not 5432. Driver location sign is off by 2km as well.

Oops! Thank you - I must have gone number blind by that point. Fixed now.

frediculous_biggs 3 August 2017

Interesting that the A20(M) is listed. I wonder if that is used for either the inner or outer carriageway of the M20 through Maidstone, or is not used at all, but still assigned.

Gabriel 5 August 2017

Roadside emergency telephones seem to use the same system of numbering (although they exist on more roads than this list).

Gary L 11 August 2017

I could show you the wonderful world of LRP's (those little two dots you see every so often in lane 1 of a trunk road) and Network Referencing if you wish??

CrashedFiesta 17 August 2017

Hello. I've been doing motorway stuff a long time too and, unfortunately, your photo showing the MS3 is a bad one to pick.

You see, the location is clearly an ALR (All Lane Running) area which means that the signals (also known as AMIs - Advanced Motorway Indicators) that are on the gantry in the background will always show mandatory signals. The MS3 in the foreground can only show advisory signals i.e. they have amber lanterns rather than a red ring. You don't mix the two so the MS3 will never show a signal aspect, only text. It's a 'strategic' sign which will be used primarily by the NTOC (National Traffic Operations Centre) in Quinton to tell you about problems you might encounter a few miles away...

Sorry to be pedantic. ;)

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