Imperial on the surface, metric beneath

Following on from his article about the far-reaching influence of the measurement units used on road traffic signs, Ronnie Cohen now takes a look at the signs themselves and the roads they complement.

Despite our roads being predominantly signed in imperial units, they are built and designed exclusively in metric units and have been since the early 1970’s and, yes, that even includes the digits and letters showing the imperial units. Take the 40 mph roundel as an example.


Despite the fact that the roundel shows a speed limit of 40 miles per hour, the permissible height of the sign is defined in millimetres as shown on the left in the image above. It is not only the height of the sign that is defined in metric units. The proportions and forms of letters, numerals and other characters for use in road markings where the character height is between 280 mm and 700 mm (inclusive) are defined in millimetres with the following limits:

  • Height of letters: 280 min, 700 max
  • Horizontal thickness: 35 min, 87.5 max
  • Vertical thickness: 35 min, 87.5 max

The same is true of road markings. The letters, numerals and other characters for use in road markings showing speed limits in mph where the character height is 1600 mm must use a horizontal thickness of 70 mm only and a vertical thickness of 200 mm only. For road markings, the height and width of the oval and the height of the digits are all also specified exclusively in metric units.

The table of minimum and desirable longitudinal clearances, including exit clearances, are defined exclusively in metres. The minimum headroom to be provided at temporary structures is defined exclusively in metres. The minimum lane widths road contractors must provide depends on the expected type of usage. These widths are defined exclusively in metres. The distances between sites for standard works and works with relaxations are based on the speed limits of the affected roads. They are specified exclusively in kilometres. Here are some instructions given to road contractors about roadworks:

  • “Speed restrictions should extend to a point, 90 m for dual carriageway roads and 45 m for one-way single carriageway roads, beyond the last cone of the temporary traffic management arrangement.”
  • “In general, a temporary speed limit should not be introduced where the length of restriction would be less than 800 m on dual carriageway roads and 400 m on single carriageway roads, measured between the initial speed limit sign and the sign indicating the end of all restrictions.”

A table showing the spacing of signs and taper length at survey sites for different speed limits give the following only in metres:

  • Siting distance apart of signs
  • Length of lead taper
  • Minimum clear visibility of sign to a particular diagram

The Traffic Signs, Regulations and General Directions (TSRGD) and the Traffic Signs Manual show graphics for road contractors exclusively in metric. For example, minimum and maximum lengths of sections are shown only in metres. Graphics show distances in exact intervals of 100 m but signs show the same number of yards.

The UK Department for Transport (DfT) gives road contractors the following principles for approach signing:

  • a “road works” sign (7001) with supplementary plate “1 mile” (572) placed on the near side 1600 m in advance of the works lead taper
  • a “road works” sign (7001) with supplementary plate “800 yds” (572) placed on the near side 800 m in advance of the fi rst cone of initial lane closure
  • a “road works” sign (7001) with supplementary plate “400 yds” (572) placed on the near side 400 m in advance of the first cone of initial lane closure
  • a “road narrows” sign (517) with supplementary plate “200 yds” (572) placed on the near side 200 m in advance of the first cone of initial lane closure

You can also find buildings, cars and televisions are all made exclusively in metric units. Despite the fact that no imperial units are used to make them, they are often marketed to customers in imperial units.

Needless to say, this kind of nonsense is found nowhere else in the world. And, although the cost of conversion is usually given for the reason for persisting with it, one wonders if short-sightedness and lack of vision by successive UK transport ministers is really to blame.

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3 Responses to Imperial on the surface, metric beneath

  1. Stan says:

    It isn't only the DfT who display this madness.

    It us all too common that short distances are spoken of in metres and then longer distances in miles.

    In the UK we don't have any system at all. People seem, commonly, to think of length/distance in millimetres, inches, feet, yards, metres, miles. For weight it is grams, ounces, pounds, "kilos", stones and tons.

    When measurements are used for comparative purposes only it doesn't seem to matter much what they use in any given situation. But, of course when it comes to more sophisticated uses then it certainly does matter and the advantages of the metric system are overwhelming. The trouble is a substantial proportion of the UK population seldom do go beyond simple comparison and it is they who are patronised by the likes of transport authorities and the media.

  2. Daniel Jackson says:

    That very same speed limit sign anywhere else in the world that uses Vienna Convention signs, the 40 would be understood as kilometres per hour. The Vienna Convention should insist that when this style of sign is used for speed, kilometres per hour would be the intended meaning. The whole idea behind these signs is for universal understanding of road rules and restrictions independent of language. So, when a sign has two meanings, confusion results.

    How many foreigners have driven their own cars to the UK and mistook the sign to mean kilometres per hour and obstructed traffic by going too slow and conversely, how many English have gone to a metric country and drove excessively fast thinking the signs meant miles per hour?

  3. BrianAC says:

    Surely the "cost of conversion to metric" is the wrong way round?
    What the phrase should read is the continued and totally unnecessary cost of "conversion from metric to Imperial" for road construction (and others).
    This presents a daily, even hourly costs to every project ever undertaken in the UK for quite a number of years.
    Just why this is never addressed is anyone’s guess.


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