Odd British Measurement Usage in the Transport Sector

Some aspects of measurement usage in the British transport sector appear to be uniquely British with no known parallel across the whole world. The British really are out of step with the modern world when it comes to transport measurements. The British transport sector is one of the last imperial bastions in the UK thanks to DfT policies.

The British are virtually unique in not allowing metric units on almost any traffic signs. Metric units are banned from official speed and distance traffic signs. The only legal metric speed limit in TSRGD 2016 is a distinctive diamond-shaped sign for tram drivers and nobody else! It is probably the only country in the world to use yards on its roads (the Americans tend to use feet on their roads). Even safety signs in tunnels show distance to exits in yards only. Successive British governments have required the metric system to be taught in UK schools since 1974 yet the same governments have forbidden metric units from most road signs.

The Welsh must show Welsh “llath” alongside English “yards” for short distances and Welsh “milltir” alongside English “miles” for long distances, either on the same sign or on separate signs. Is Wales the only nation that has a legal requirement to show non-metric units in more than one language? If they used metric, they could use the “m”, “km” and “km/h” international metric symbols. Unfortunately, imperial units have no common language-independent symbols so the Welsh are stuck with verbose signage.

Strangely, the British use “m” for metres and miles. This leads to some bizarre usage:

Is this supposed to mean 0.2 metres and 0.3 kilometres? This usage is no doubt influenced by DfT use of “m” for miles. This footpath is intended to show a distance of 0.2 miles, not 300.2 metres! One odd fraction of a mile is a third but it does not fit easily with use of yards and miles. There are 1760 yards in a mile but 1760 is not divisible by 3. A third of a mile is exactly 586 yards 2 feet! Some footpath signs show only miles, some show only kilometres and some show both. As a result of DfT convention, you might come across a sign with text like “16 km 10 m”, strangely intending “10 m” to be interpreted as 10 miles and not 10 metres. This muddle extends to maps. While ordnance survey maps are exclusively metric, commercial maps tend to use both metric and imperial units.

Only signs telling drivers who cross the border between Northern Ireland and Republic of Ireland that speed limits are in different units (ROI uses km/h and NI uses mph for speed limits) mark the border between the UK and Ireland. Apart from that, the border is invisible. Is this the only international border marked by signs informing drivers about a change of measurement units?

All road design and construction in the UK is carried out exclusively in metric units. The only thing that is imperial about our roads is the display of imperial units on our traffic signs and road markings. Road contractors do not use the imperial system at all except at the final stage, when they have to convert all their metric measurements to imperial units for display on traffic signs. Even the bridge heights, bollard widths and distances between locations on our roads are all measured in metric units although the traffic signs show these measurements in imperial units. Even the dimensions and thickness of the digits showing these imperial measurements and all other parts of the traffic signs on which they appear were designed and manufactured exclusively in metric units. The same is true for road markings. This all comes from the DfT Traffic Signs Manual. Isn’t that daft?

Roadwork signs showing multiples of 100 yards are placed at multiples of 100 metres:

Drivers are not told that the number of yards shown on “roadwork ahead” signs is really the number of metres.

The Highway Code expresses stopping distances in metres, feet and car lengths, visibility information in metres and highway regulations in metres and feet. It also says that drivers need to be able to read number places from a distance expressed only in metres. Some parking restrictions and recommendations are also expressed in metres and feet.

None of these measurements are authorised for distance signs, which must be shown in yards, miles or fractions of a mile.

Miles and kilometres are both mandatory on British speedometers. Kilometres per hour have been required on British speedometers since 1977, presumably in preparation for the metric changeover, which has never happened and there are still no plans for it. This was postponed indefinitely in 1970. Traffic signs are still almost exclusively imperial over half a century later. Progress in introducing metric units on traffic signs has been extremely slow.

Vehicle dimension signs come in metric-only, imperial-only and dual varieties. The metric-only ones tend to be used by the private sector, especially at petrol stations and private car parks. Despite mandatory dual vehicle dimension signs since the TSRGD 2016 came into force, there are still plenty of imperial-only ones around. This variation in units has produced a bewildering range of designs for vehicle dimension signs. Dual signage can involve metric and imperial units on a single sign or on separate signs.

While ferry companies ask drivers to give their vehicle dimensions in metres and their vehicle manuals show their vehicle dimensions in metres, in-cab vehicle height indications must be in imperial units by law. This shows the disconnection between government requirements and private usage. While the DfT insists that metric and imperial units must appear on all new and replaced vehicle dimension signs, its website uses only metric vehicle dimensions to describe the maximum vehicle sizes in government legislation (e.g. sizes of motorhomes and trailers, vehicle specs in driving licence codes and categories).

The DfT insists that low bridge signs like the one shown above uses 4 measurement units but 1 is enough. Only metres are needed to show bridge height and distance to a hazard. Low bridge signs in Wales require Welsh and English versions of the text. If metres were used, a number followed by the international language-independent “m” symbol would be all that is needed to display the distance to the bridge.

Fuel consumption is measured in litres per 100 kilometres, kilometres per litre and miles per gallon. Carbon dioxide emissions are expressed in grams per kilometre. None of these measurements fits well with fuel sales in litres and distance signs in miles. The British cling onto miles per gallon for fuel consumption despite the fact that they have not sold anything by the gallon for more than 30 years.

The DfT has an unofficial de facto “metric for professionals, imperial for general road users” policy. They tend to use the metric system exclusively for tram and light-rail drivers, the emergency services and road contractors. Traffic signs and road markings for general road users are almost exclusively imperial with limited use of metric units.

Until the introduction of the new metric social distancing signs, the only traffic signs that were purely metric were the weight restriction signs. The only measurements where the DfT consistently and exclusively uses metric units are the ones related to weights. The DfT only uses kilograms and tonnes for weight throughout its website, forms, traffic signs and publications. Why has the DfT been unable to apply this consistency with other measurement units?

Older traditional railways use miles per hour speed limits and chains and miles for distances but modern rail systems (e.g. trams, DLR and other light rail systems) use km and km/h.

On the government’s Speed Limits webpage, speed limits are given in mph for a wide range of vehicles on different types of road (built-up areas, single carriageways, dual carriageways and motorways). These vehicles include cars, motorcycles, car-derived vans, dual-purpose vehicles, motorhomes, motor caravans, buses, coaches, minibuses and goods vehicles.

However, there are some notable exceptions and inconsistencies. Speed limits for other types of vehicles are given in km/h. There are licence categories for mopeds that are based on metric speed limits. The AM category allows you to drive 2-wheeled or 3-wheeled vehicles with a maximum design speed of over 25 km/h but not more than 45 km/h. This includes light quadricycles with a top speed of 45 km/h. The P category allows you to drive 2-wheeled vehicles with a maximum design speed of over 45 km/h but not more than 50 km/h. The Q category allows you to drive 2-wheeled and 3-wheeled vehicles without pedals with a maximum design speed of no more than 25 km/h. Bizarrely, the licence categories for mopeds specify speed limits in km/h whereas the recent e-scooter legislation legally defines e-scooters that meet several criteria, including one with an mph speed limit. Go figure!

From 2008, the practical speed limits for goods vehicles and buses on UK motorways became 90 km/h and 100 km/h respectively because of the speed limiter regulations which came into force in January 2008. As these large vehicles must have speed limiters fitted, these regulations override the official speed limits for these types of vehicle. They are incompatible with the national speed limits for these types of vehicles on some types of road.

The speed limits are also incompatible with the digital tachographs that must be fitted to some large vehicles. Digital tachographs are instruments that are used to record driver and vehicle activity to ensure compliance with drivers’ hours rules.


Digital tachographs record and display distances and speeds exclusively in kilometres and kilometres per hour. Many goods vehicles exceeding 3.5 tonnes, and some coaches, are legally required to be fitted with a digital tachograph.

Another odd feature about the mixture of metric and imperial speed limits is the use of strange-looking mph speed limits used in certain contexts. For example, there are off-road tests for motorcycles and mopeds. Here is what the DfT website says about these tests:

“For the hazard avoidance and emergency stop exercises, you must ride at a minimum speed of:

  • 19 mph on a moped
  • 31 mph on a motorcycle”

I presume that these odd-looking mph speeds are supposed to be conversions for 30 km/h and 50 km/h. These kilometre-based speed limits cannot be found anywhere on British roads and are incompatible with the ubiquitous use of mph speed limits so these tests cannot be performed on British roads and must be performed at off-road testing centres.

One page of DfT advice on becoming a fully qualified lorry or bus driver tells you that “you do not need the full Driver CPC (to become a qualified lorry or bus driver) when driving within 62 miles (100 kilometres) of your base” and “do not need the full Driver CPC if the vehicle is limited to a top speed of 28 mph” (equivalent to 45 km/h). Why do they use these odd mile-based figures? Is it because they come from EU directives that use round metric numbers?

All these absurdities arise from the British attempt to use two incompatible systems of measurement. Miles on roads are now used in the UK, the US and several smaller countries. Most of the smaller countries are UK or US territories or have close ties to the UK or the US. I am not aware of any other countries that follow British practice in measurement usage in the transport sector. It seems to me that these practices appear to be uniquely British. How does British practice compare to usage in other countries that still use miles on their roads?

Most references to the DfT website used in this article can be accessed from https://www.gov.uk/browse/driving web page. Other sources are listed below.

6 thoughts on “Odd British Measurement Usage in the Transport Sector”

  1. Thanks, for your article, Ronnie. The DfT really is intensely annoying in its attitude.

    So far as road traffic is concerned, I have a number of gripes. One is the following.

    Most (if not all) cars sold in this country are changeable to suit the driver. One change could be the car’s language. For example, because I decided to learn spanish, I switched the car to speak spanish, both visually and audibly. Another change is to use km instead of miles.

    Now, fifteen years ago (eg my wife’s ’08 Renault Twingo) things were simple: if you changed from mph to km/h the odometer and the trip meter, as well as the speedometer, changed from miles to kilometres, or vice versa. In 2017 my new Kadjar could be switched, and everything, including the up-to-date equipment changed. Except that I had to pay Renault £80 to download the software which made this happen.

    My current car is a ’72 registration with GPS navigation and all kinds of gizmos. Switching from imperial(ist) to metric works only on the speedometer and the sat-nav. Output from the car’s computer, even with the display in spanish, insists on mi (not m), mpg and mph for the various data. I find the imperiousness of forcing my car to using “millas”, which no Spaniard would use, unacceptably arrogant. The dealership kept the car for the better part of a day in a vain attempt to download software which would free all aspects to be expressed in metric units.

    This is an utter nonsense, since there cannot be many places left in the Kingdom, where petrol and diesel can be bought by the gallon. And, as we well know, if this is for the convenience of the US market, our gallon would have to be recalibrated to their Customary gallon anyway.

    There is one slightly bizarre, and somewhat amusing side to this. The dashboard display helpfully shows the current speed limit in miles per hour, using a standard speed limit sign. Also helpfully it adds a little box to the speed limit sign, giving the equivalent in km/h. Except from time to time, for reasons which I do not understand, the system gets confused.

    The first time that it happened was on M25 near Clacketts Lane service area. The variable speed limit was set to 50. However, what was displayed on the dashboard—and, incidentally, on the sat-nav—was 80. On the dash display it helpfully informed me that this was 130 km/h.

    It’s happened several times since, but the most disturbing are the occasions when I have been stationary at traffic lights in the middle of our town—limit: 30mph—, and the dash shows me 100, which it then converts to 161 km/h. I haven’t tried it yet.

    I’m tempted to start a guerrilla group to go around and sabotage imperial signs with metric overlays. Any takers?


    Nicholas KERR nicholasian@mac.com


    Liked by 2 people

  2. Transport includes not only road, but rail, air and water transport.
    British rail transport is in a similar mess as is evident in road transport. Railway vehicles are designed and built using metric units and the standard rail gauge is officially 1435 mm. However speed limits and distances on legacy routes (ie any railway that was spared by Beeching in 1965) are still in miles and miles per hour. However routes that have been built since then (including the Channel Runnel Rail Link and HS2) and all British tram and metro systems in metric units .
    There have been two drivers behind the conversion to metric units. Firstly, the driver behind the use of metric units in railway vehicle construction was part of the general change-over to metric units in the engineering industry in the 1970’s. The conversion of speeds and distances on existing lines however required the cooperation of the railway unions. As is well-known, the railway unions are never slow to seize an opportunity to squeeze higher wages for their members and I believe that this has been one of the reason that full metrication of the railways has been held back.
    The second driver is the much-hyped “digital railway” (ie European Train Control System or ETCS) which will in time replace the myriad of signalling systems found on British railway routes. It is of pan-European design and has been exported to many countries outside Europe including Australia, India and Brazil. Unsurprisingly it is designed to work entirely in metric units, though there is a bodge for use in the UK which will enable display units to switch between km/h and mph, depending on the route on which they are operating. The first route to use ETCS in the United Kingdom was the Cambrian Line. This line was selected as it signalling needed replacing during the first decade of this century and the line itself is largely stand-alone. From an engineering point of view the use of ETCS on this line was over-kill, but this was essentially a trial run to gain experience if the use of ETCS in a British environment.
    One of the more bizarre aspects of post-1970 overhead electrification of legacy routes is that the identifiers on the catenary masts give the mast location in kilometres from the reference point while the equivalent markers for the railways themselves use miles. Also, if the Rail Accident Investigation Branch (RAIB) needs to report on an accident, the location will often be described in the form “50 metres before the 45 mile marker”. The reason is that the 45 mile marker was there before metrication and so, from the point of view of the RAIB, the “45 mile marker post” is purely a refence point.

    Liked by 2 people

  3. Here is one source (not vetted) for those countries that post road speed limits in miles per hour vs km/h:

    I searched for images of speed limit signs in Myanmar and got some photos with just km/h and some that posted both mph (with a sensible number) and also km/h (where the number was clearly a conversion from Imperial). Anyone close to traveling to Myanmar to bring back the real story there? 😉

    I also tried Liberia and came up empty handed. Same question about any travelers thinking about visiting West Africa. 😉

    Aside from the USA and the UK, all those places still posting speed limit signs in mph are tiny and/or unimportant on the world stage. But the UK truly is a world class muddle in this regard, sadly. 😦

    Liked by 1 person

  4. Ezra: you’d have to laugh or you’d cry! The ‘professional’ blog to which you post a reference contains a hotchpotch of ways of writing kilometres per hour, including the most common error kph. Thankfully it does include the correct symbol km/h too, but … professional?!


  5. I was interested to read of @mizeki’s experience with his Kadjar, I have a 2017 Astra (the last of the GM models) which switches everything really easily though I have to be careful to switch back to Imperial if my car goes into the garage because sometimes they switch it back themselves, other times they leave it as it is and the documentation gets messed up (having to get an MOT re-issued to show the correct mileage was a nightmare). And all to maintain consistency in the service history.

    I do think that some manufacturers are only really paying lip service these days to the ‘requirement’ that equipment can be changed, I’ve seen cars with speedos ONLY marked in MPH (the BMW Mini being one of those) and I suspect that post-Brexit less time will be spent on this feature and I’m actually dreading having to replace my current car partly for that reason.

    Continuing the motoring theme I also found that used car sales don’t require fuel consumption to be shown in l/100 km so having to navigate MPG after having not used it for so long was a real pain.


  6. Martin Vliestra wrote: “Transport includes not only road, but rail, air and water transport”

    Yes, of course, and thanks for that post. But operating vehicles in rail, air and water transport is a professional job, and you will have to adapt in a professional capacity to non-standard measures where they are used, as a train driver, pilot, captain, etc. The same cannot be said of private individuals driving their cars or other road vehicles on the roads. They are not “working” and should not have to be expected to cope with non-standard measures of speed and distance. Many much older drivers will have learnt the imperial measurements as children, but younger people today (basically anyone under 50!) should be able to drive and use the roads using the metric measures they learnt at school. What is the point of an official system of measurement if it’s not used?

    Liked by 1 person

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