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A PHYSICIST WRITES . . .
(January 2009)
As I drive along, I do try to notice and make sense of anything unusual (which, after all, is how these columns often first take shape!). But there’s something I became aware of only gradually, last year. I’m referring to the blue signs that you now see every few hundred yards along motorways, and on some other roads too I believe.
When I first observed them, their message looked to me like some private code for use by the highway authorities, and so I paid little attention to them. Then I noticed that the bottom number varied like a measurement of distance. But it changed too much, from one sign to the next, to be in miles. Finally I realized it was measuring kilometres — which makes it surely the most blatant example of imposed metrication across the country (except for measuring temperatures in Celsius, not that I object to this).
Hunting for an explanation of these ubiquitous markers, I found one at the very end of the 2007 edition of Know Your Traffic Signs, almost a footnote: “They show the motorway or road number, the carriageway identifier and a distance reference. In the event of a vehicle breakdown or other emergency, the exact location can be identified quickly.” Well, how about that! Couldn’t these Driver Location Signs, as they are called, have been announced rather more widely? Certainly I’ve never seen publicity for them, though you might have expected plenty if they were being installed for general use (and at considerable cost too, no doubt).
So I looked at them harder and investigated them further. At the top of the sign is a reminder of the motorway number: apparently, not all drivers phoning the emergency services can remember which road they are on. The second line consists of just one letter, usually A or B. On motorways radiating out from London, A marks the outward carriageway and B the returning one. On the M25, A takes you clockwise and B anti-clockwise. I’m not sure what the rule is elsewhere. Other letters are used on slip-roads.
And then below this we get the ‘distance reference’ — a number that can run into the hundreds, and which ends with a digit after the decimal point. Now if the authorities insist, I am prepared to accept that this distance (from wherever it’s measured) is indeed accurate to the nearest 0.1 km. But aren’t the decimal point and that final digit rather likely to hinder you, as you desperately try to remember and report the whole location code when phoning the emergency services? It would have been better if more of the signs had been placed exactly on the kilometre!
In fact, according to the Highways Agency website, Driver Location Signs were introduced mainly to assist passengers (and, I suppose, hands-free drivers) in reporting incidents by mobile phone without needing to stop. But did you know that if you do have to stop in emergency, you will actually find the decimal distance (with its decimal digit printed below the whole number) on every marker post?
That was another surprise to me. I knew the posts were 100 metres apart and had some sort of identification on them, but I had no idea that they already calibrated our motorways, kilometre by exact kilometre (and have done so for 30 years). The puzzle is why the Highway Code still advises you to trudge to the nearest emergency telephone rather than use your mobile — though admittedly the telephone does transmit its location automatically for you.
As for the distances themselves, it seems they are measured either from the start of the motorway or from the centre of town (eg, London), whichever is more appropriate. It’s understandable, perhaps, that the two carriageways are always measured from the same end, though the result is that on the ‘B’ side the decimal number decreases (rather confusingly, would you agree?) as you drive onwards. But you would have thought that the anti-clockwise circuit of the M25, at least, could have been measured in its own direction.
What I really want to know, though, is whether the marker posts on the M25, striding out in parallel from somewhere near Dartford, were allowed by the constructors to get progressively out of step between the two carriageways, because of the difference in their circumference!
At 70 mph it’s not easy to investigate this, so let’s suppose the motorway is on average 35 metres wide, including the hard shoulders. Then the outer (clockwise) hard shoulder ought to be just over 200 metres or 0.2 km longer than the inner one, regardless of the diameter of the motorway and how it twists as it circumnavigates London. Now if someone happens to be visiting Thurrock (where the M25 ‘ends’, before crossing the Thames labelled the A282) and could find time to examine the motorway and check on this...
While getting to grips with the blue signs, I came across the official Traffic Signs Manual. Aimed at councils and other bodies responsible for road signs, it contains a huge amount of advice, mostly sensible. For example, each of those ‘priority’ signs with the pair of vertical arrows — the round sign (give way to oncoming traffic) and the rectangular one (go for it!) — is accompanied by a warning: Must never be used upside down in an attempt to imply reversed priority. Good, I find these signs confusing enough as it is, when I’m in a hurry.
Left-to-right reversal is allowed for particular signs, of course. Even the bicycle-in-a-triangle may be printed either way round, to warn you of cyclists emerging from whichever side of the road is more likely than the other. I wish you luck, though, in noticing and making sense of that subtle difference, at speed.
Peter Soul
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