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A PHYSICIST WRITES . . .
(February 2013)
In the last issue of the Advanced Driving magazine, the following question was put to “The Experts”: My daughter recently received notice of prosecution for contravening a red traffic light ... she is adamant that the duration of amber was insufficient to stop safely or cross the junction, before the change to red. Are there official requirements for the duration of the amber light?
The (unsigned) answer was, I felt, in the style of Peter Rodger, Chief Examiner of the IAM: The length of the amber light is of less relevance than at first it appears, since you must stop on amber unless it would cause a crash. The driving trick is to see how long a light has been green, because the longer it was green, the sooner it will be red. Would you say this was helpful? Me, I felt obliged to send in some comments:
[I hope I won’t confuse readers by making additional comments here (in square brackets) on the comments (in quotes) that I submitted. The latter didn’t appear in a subsequent issue of Advanced Driving, so I can’t claim that they have been officially approved!]
“The simple answer is that the duration of amber alone is three seconds (in the UK). Also, calculation from this figure plus the stopping distances in the Highway Code shows that if, when amber appears, you are closer to the line than the theoretical stopping distance, then you can always get your car over the line before amber changes to red – at any speed below 65 mph. Above this speed (or below it, in poor conditions) you might find you can neither stop at the line nor cross it in time.”
[...which makes duration of amber seem highly relevant, as it would significantly affect the 65 figure. By the way, I notice that this is less than the 70 mph I calculated back in June 2005 for this column – possibly because I’m now allowing for the fact that really, your whole vehicle ought to be over the line before the lights go red! The Highway Code stopping distances haven’t changed since 1946, of course, but that’s another story. Anyway, to continue my comments...]
“What’s puzzling is the answer that was printed: ...you must stop on amber unless it would cause a crash. This isn’t quite what the Regulations say, which is that you may cross the line on amber, but only if you cannot safely stop behind it (as explained by Peter Rodger in the Winter 2007 issue, I think it was). No need to worry about trying to predict a crash, then, when amber shows: just decide if you can safely stop behind the line or not. Or better still: at every approach to traffic lights, simply judge where your ‘point of no return’ is, and from there on ignore the lights and concentrate on possible hazards.”
[...yes, Peter R made the regulation crystal-clear in 2007 (replying to a question from me, in fact). Yet now in Advanced Driving we’re being advised to obey what is effectively a precis of Rule 175 of the Highway Code, which I see as being different. As for the statement that the longer the lights have been green the sooner they will be red, it doesn’t necessarily follow: they might stay green until triggered by cross-traffic or by pedestrians, or the change might be delayed by the approach of your own vehicle. And you can consider the situation from another angle...]
“I sometimes face a dilemma at familar intersections if clues are telling me well in advance that a green light is going to change to amber just before I reach the line. What should an advanced driver do: safely press on in order to make good progress – or anticipate the amber, slow down early, and safely stop as per the Regulations?”
[That’s the end of the comments I submitted, and of this topic.]
I’ve read some startling items of physics-related news recently. One was also traffic-light related (well, nearly): first a type of lens was devised for spectacles to help medics see faint blood-vessels in the skin, by enhancing their apparent redness. Then the specs (called Oxy-Iso) started being sold as “amplifying your perception of health, distress, and emotion signals on the skin.” And lately it’s been found they help to overcome red-green colour-blindness! But alas, they’re no good for driving, as they mask yellow and blue tints, so drivers who have a problem at traffic lights must continue relying on the position of the light.
Mains hum at 50 cycles per second has always been an annoyance for sound engineers, who have to eliminate it as they try to produce perfect recordings of music etc. But it has also become a weapon against crime. The frequency is actually always varying very slightly, either side of 50 – and the Metropolitan Police Forensic Laboratory now has an exact record of the variations going back seven years (analagous to the pattern of changes in tree-ring width that’s used to date old wooden items). So when a phone-call recording, say, is offered as evidence, its faint background hum can be analysed, and its time, date and integrity confirmed!
Something whose integrity is vital, for physics if not the whole of science and technology, is the basic set of units for mass, length, time, and one or two other quantities. Of these, the unit of mass (or weight, if you’re not familar with mass) is the only one still to have an ‘arbitrary’ definition, namely the mass of a kilogram block of platinum stored in Paris since 1884, though there are 40 official copies around the world. But now it’s been discovered that they are all getting heavier. Logically then, we each of us weigh a bit less (in terms of kilos) than we used to! Astonishingly, the reason is that the platinum surfaces are being contaminated by hydrocarbons. These are now having to be very carefully removed. So it’s back to the diet, some people...
Are you a smoking driver? Here’s an offer for you: would you like to test a small disposable container for your cigarette-ends which even extinguishes them as you drop them in and close the lid? It’s supplied by an anti-litter group that I belong to, called Zilch. Take a look here if you’re interested:
www.surveymonkey.com/s/ZilchIn-CarBinTrial.
Peter Soul
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