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Blog: What's in a number?
How our lives are dominated by numbers yet we somehow get it wrong

5. How high is a bus?

When I go away from home, I sometimes travel by bus. These buses carry notices visible to the driver indicating the height of the vehicle. Of course the driver needs to know the size of the bus to ensure that it can fit under any bridges (see my first blog item "How high is a bridge?"). As we have seen, bridges are slightly higher than the advertised size, but I have no reason to suppose that the size of a bus marked up near the driver's seat is anything but reasonably accurate, but who knows?

Some of you, though, might realise my disappointment when I saw on some of the Plymouth buses 13' 10", 4216MM. Not only is the Imperial measurement given first, contrary to government guidelines, but the wrong abbreviation for millimetres is given. Further, there is no space between the metric distance and the unit contrary to recommendations. In short, this vaguely obeys the rule of quoting the height of the bus in metric but it doesn't obey the spirit to the extent that nobody is encouraged to use the metric when the Imperial measurement stands out.

Another thing is that I doubt very much if the bus is really 4216 mm high. That is clearly a direct conversion from the Imperial measurement (which in fact comes to 4216.4 mm). If we assume that the bus has been measured only in Imperial units, it could easily be 1 or 2 inches away from the advertised 13' 10". In other words, the true height of the bus might be anything from 13' 8" to 14' or 4.17 to 4.27 m. Perhaps the engineers have been at it again, and added another 2" for good measure. That is the trouble with "engineering" measurements. If you substract a small distance from a bridge height for "good luck", should you also be consistent and add to the height of a bus just to make doubly sure? If so, where do these "engineering corrections" ever end?

In any case, the quoted metric measurement is nonsense because of the range in the possible height of the bus. In other words we can only be reasonably certain that the height is somewhere within a 10 cm range, 100 times the 1 mm implied uncertainty in the quoted measurement. Of course it only takes a few coats of paint (or a few near misses!) or a flat tyre to change the height of the bus by 1 mm. This false accuracy by which the bus height is quoted does nobody any good. Unfortunately, similar false accuracy occurs in a lot of places. It encourages people to convert from one system to the next rather than stay using metric entirely. Numbers also come out awkwardly and unmemorable, like our 4216 mm bus. It is a bit like somebody speaking a foreign language by translating every word directly. That is not how people communicate across languages as it is too time consuming and an obstacle to fluency.

Another issue is that on the roads, widths and heights are given in metres and tenths such as we have seen in my first blog post, e.g. 3.4 m for the height of a bridge. Faced with a measurement on the sign in metres and the height of the bus in millimetres, is the driver going to want to divide by 1000 for proper comparison? I suspect not. Apparently, all road signs indicating width or height restrictions in Britain now have to have both Imperial and metric measurements, at least when the signs are renewed. Although this modest progress towards a common set of units throughout the land (as required by the Magna Carta 800 years ago, and not so far rescinded!) is to be commended, it is a typical British approach of doing things slowly and deliberately at a snail's pace.

So, to return to the notice on the bus, it is horrible from a number of viewpoints:

  1. Imperial incorrectly given first.
  2. No space between metric measurement and unit.
  3. Incorrect abbreviation for metric unit.
  4. False accuracy implied in the metric measurement.
  5. Height given in millimetres when metres would have been preferable.

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Website revised by John Austin, 29/1/2015. © Enigma Scientific Publishing, 2015.