Highway Code conversion confusion

Ronnie Cohen looks at consequences of the UK’s measurement muddle for The Highway Code.

One major issue that arises from the UK measurement mess is the seemingly random and illogical mixing of units, of which The Highway Code provides a good (or should that be bad?) example.

Recently, the Department for Transport (DfT) held a public consultation about proposed changes to The Highway Code. I responded to the consultation and suggested some changes. Let’s hope that there will be some advances when the next edition is published. There is room for improvement but issues with measurements will remain until the UK completes its metric transition.

Speed limits in The Highway Code are remarkably consistent. They are quoted in miles per hour (mph) followed by kilometres per hour (km/h) in brackets. I praise the DfT for using the correct km/h symbol throughout the Highway Code. However, even for speed limits, there are a couple of notable exceptions. In Section 273, speed limits are expressed in mph only. In this section, it says “On leaving the motorway or using a link road between motorways, your speed may be higher than you realise – 50 mph may feel like 30 mph.” In an Annex, only km/h is used for motorcycle licence information but mph followed by km/h in brackets is used for moped licence information.

The only measurement that The Highway Code consistently uses metric exclusively is weight. Wherever weights appear, they are expressed in kilograms or tonnes. No conversions are given for any weights.

Usage of units for short distances is really inconsistent. In some places metres are used exclusively, and in others metres are given with conversions.

You can find exclusive use of metres for:

  • Reading a number place from a distance (Section 92).
  • Height of tramway overhead wires (Section 307).
  • Distance between you and the car in front when stopping in a tunnel (Section 126).
  • Vehicle lengths (Vehicle markings section, Large goods vehicle markings subsection)
  • Load or equipment overhang (Vehicle markings section, Project markers subsection)

You can find use of metres followed by conversions for:

  • Goods vehicle travelling less than a certain distance (Section 99).
  • Child’s height for seat belts/child restraints (Section 100).
  • Typical Stopping Distances chart (Section 126).
  • Average car length (Section 126).
  • Reduced visibility (Section 226).
  • Parking proximity to a junction (Section 243).
  • Distance from a junction for some vehicle types (Section 250).
  • Minimum distance behind broken-down vehicle to put a warning triangle (Section 274).
  • Usual clearance for overhead electric lines (Section 292).

One use of height information that I find most incomprehensible is in the table in Section 99. In this table, the third row uses metres only for a child’s height but the following row uses metres followed by feet in brackets. If you wanted to give only a single conversion, you would surely provide it for the first instance, not for subsequently.

One use of yards in the text appears for the interval between countdown markers at the motorway exit in the Traffic Signs section. They appear without any conversions to other units. It reflects the regulations in The Traffic Signs Manual for countdown markers to be placed at 100 yard intervals. By contrast, emergency marker posts must be placed at 100 metre intervals on motorways and road works signs must be placed at 100 metre intervals as well even though they show the same number of yards to roadworks. Why is there inconsistent usage of units for the placing of different types of signs?

There are other metric measurements without conversions:

  • Breath and blood alcohol levels in micrograms per 100 millilitres (Section 95).
  • Capacity of motor vehicles in cubic centimetres (Section 253).
  • General motorcycle information in cubic centimetres and kilowatts (in Annexes).
  • Motorcycle licence information in cubic centimetres and kilowatts (in Annexes).
  • Moped licence information in cubic centimetres (in Annexes).
  • Tread depths of tyres in millimetres only (Vehicle safety, maintenance and security section).
  • Compression information for first aid in centimetres only (First Aid on the road section).

It is clear that the DfT expects readers of The Highway Code to be familiar with millimetres, centimetres and metres. As metric measures have been taught in primary schools for almost 50 years, we would expect this. So why does the DfT insist that road distances must be shown only in yards and/or miles and that restriction signs cannot be shown only in metres? Lengths and heights on other signs must be shown in feet and inches as well as metres. In The Highway Code, metres come first followed by imperial units. Why not just metres?

10 thoughts on “Highway Code conversion confusion”

  1. According to the Highway code the safe stopping distance at 70 mph is 96 metres or 315 feet. (Since when were road distances in the UK measured in feet? We are not a US colony even if we have a Prime Minister who was born in the US). I know that 96 metres is the approximate length of a football or rugby field, but when one is sitting behind the wheel of a car on a motorway, one gets a totally different perception of distance.
    There is however one very good approximation to the safe stopping distance – distance marker posts are spaced at 100 metre intervals (which is about 96 metres plus one car’s length). It is a pity that the editors of the highway code did not link this up with travelling at 70 mph. Likewise, travelling at 50 mph, the safe stopping distance is given as 53 metres which is about half the distance between pairs of distance marker posts.

    Liked by 1 person

  2. As per usual the previous governments that failed to convert road signs to metric have left an albatross around the public’s neck when it comes to road signs and thinking in metric. This is why the BBC gave the wind speed of the latest storm to hit Britain in mph [sic] rather than km/h.
    I am again reminded of how a country like Canada, which is saddled with the American “Imperial” behemoth at its doorstep as a huge roadblock to full metrication there, can still show the way by converting road signs to get ordinary folks to use metric in everyday conversation. In particular, I have started watching a YouTube channel on public transport (not the first one I subscribed to) and saw immediately that the Canadian host (from Toronto) used “kilometers” and “km/h” quite fluently and effortlessly. Canada converted in the 1970’s and has used metric for longer distances and vehicle speeds ever since with no hint of Imperial in those contexts. Just like Ireland did and with the same effect.
    Can’t wait for road signs to be converted under a non-Tory government (at least starting with distance signs and cleaning up the Highway Code to be totally metric other than allowing Imperial speed limit signs until a conversion plan is in place for those guys).

    Liked by 1 person

  3. I too have commented in various places about the inconsistency between the Highway Code and the actual situation on the highway, but Ronnie has really gone through it with a fine tooth-comb.
    May I say that I entirely agree with Ronnie and the two subsequent posters.

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  4. When the UK was deciding to metricate in the ’60s and ’70s, why was the DfT allowed to refuse to metricate signage and even get laws passed to make it illegal to put up metric signs? The DfT should have had their refusal rejected. This made no sense other than their hope that their refusal would ignite a resistance across the economy and metrication would fail as it has done in ‘murica.

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  5. @ Daniel
    It was all down to the cost, “The costs to be borne where they fall” was the death knell of the project (apart from all the other issues). A fabricated make believe cost estimate by the then D of T made a good (bad) case for delaying tactics that worked. Technically I think it is still in the planning stage! Planning how not to have to spend the money that comes out of the DfT budget, but that will save the country probably millions a year with the knock on effects.
    Previous articles on this blog expand on that.
    A more personal opinion on my part was that metrication and the not so loose connection with the EU project was designed from the outset to fail ultimately.
    The other fact is that EU car number plates were only voluntary (as ever at higher cost for the extra symbol), instead allowing various symbols as a smoke screen lapped up by the gullible. I see now pretty much all cars sold in England have the UK union flag on the plates. It seems the cost of that patriotic symbol is OK in these times of so called hardship, even though it means the not inconsiderable cost of replacing both number plates on cars that had plain plates (50% or there abouts). I did actually refuse to buy a car that had been so modified.

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  6. The fact that the Driving Standards Agency didn’t correct an error in a conversion table in the draft Highway Code, even when UKMA pointed it out to them in the 2006 consultation, does make you wonder if they actually read the responses to their public consultations.

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  7. Brian,
    Money is always the issue, but in this case money is/was the excuse. All of the Commonwealth countries managed to do it and didn’t fret about the money. They found a way. Canada reduced the cost by simply applying adhesive stickers with the metric values over the FFU values. Ireland waited til the signs wore out and changed them on an as need basis. It took them a lot longer, but in the end all of the signs are now metric, with the speed limit signs changing in 2005.
    The fact is the DfT managers didn’t want to metricate and held back hoping metrication would be a failure as you noted everywhere else in the economy. A failure that didn’t materialise and now the angry Luddites are trying to sabotage metrication before they pass on. Maybe those still living are hoping that by holding out they can claim they were right all along and England will return to FFU with road signs leading the way. A hope that will never bear fruit.
    Odd thing in all of this is that all of the engineering, design, maintenance and even the placement of signs is done using the metric system. Why was money an issue with signs, but not with all of the behind the scenes operations? Why go half way? Why not all or nothing?


  8. Brian AC wrote “A more personal opinion on my part was that metrication and the not so loose connection with the EU project was designed from the outset to fail ultimately.”
    I’m not with you on the ‘not so loose connection’. The UK saw the advantage of moving to metric units several years before we joined the then EEC. While the idea may have been that the UK would ultimately complete the changeover once we were a member, the fact remains that the EEC/EU almost bent over backwards to accommodate the UK’s wishes to retain certain imperial units. I vividly remember Mrs Thatcher’s triumphant announcement that she had ‘saved the pint and the mile’. Road signs themselves enjoyed a long period of exemption from change and were then permanently exempted, I believe, as was the pint. I think the UK could well have sought financial assistance from the EU to complete the switch to metric road signs if it had been so minded. It would have been a major European project and fully in line with single market objectives of having a single system of measurement throughout the EU. Sadly that option is no longer available to us.


  9. I believe that the real reason metric units were not introduced to British roads was the undue influence of a group of Brexiteers who hi-jacked the anti-metrication movement, not because they had any interest in metrication, but rather to use it as a tool to further their anti-EU agenda. Due to our “first-past-the-post” electoral system, small swings in voter sentiment can have a large effect on Government popularity and as a result the British Government bends over whenever any pressure group manages to get the attention of the tabloid press,


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