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A PHYSICIST WRITES . . .

(June 2005)

This month, letís pay some attention to traffic lights. First a couple of quick questions: what is the meaning of a (non-flashing) amber traffic light? And how long does it stay illuminated? The answers, when I checked them, rather surprised me ó they are at the end of this column.

I do not recall ever seeing a sensible answer to a third question: what possible use is the red-and-amber phase (assuming youíre awake and ready to go)? You are not supposed to cross the line on the amber, but many drivers do so. If the authorities allow for this habit when setting the cross-timing of lights at junctions (as I hope they do) then surely red-and-amber becomes pointless.

Now to what I think is the central problem that you face at traffic lights: if they change to amber as you approach, how should you decide whether or not to stop? I donít remember being given advice on this.

Perhaps the obvious answer is to treat the situation like any hazard, reacting to it when it happens and attempting to making the right choice as quickly as possible. But hazards should be anticipated and anyway this seems a risky procedure to me, so I try to follow a slightly different one: I make a decision on where my minimum comfortable stopping distance from the lights will be ó and if they change to amber before that point then I brake, but if itís afterwards then I ignore them.

Suppose I pass this point of no return (no stopping, I mean) and immediately the lights change: is there a possibility that I wonít get to the line and cross it, in the three seconds before the amber changes to red? In theory, on a dry road there should be no danger of this happening, because the distance you can cover in three seconds is greater than the stopping distance (as listed in the Highway Code) at any speed up to and including 70 mph.

But if the road is slippery, or if youíre over-estimating the safe stopping distance anyway, you could get into trouble. To take an extreme example, suppose the weather is icy and you figure that the braking distances in the Highway Code need to be multiplied by three. The maximum speed at which you can safely approach traffic lights then actually drops to one-third of 70, or just about 23 mph.

Agreed, this is probably quite fast enough for driving on ice ó but the point is that if you are travelling any faster, and you decide not to stop on amber because of the distance, then you may not be able to escape the red light (unless you are accelerating) again because of the distance. Remember that, next winter!

What is the most advantageous situation to be in when red changes back to green (on a dry road)? Hereís a scenario: a car arrives at a junction while the lights are red, and brakes smartly to a halt at the line. You were alongside in the outer lane, but you braked earlier and are now rolling at a steady 10 mph towards the line.

Your timing happens to be good ó and the red-and-amber phase enables you to make it perfect, so that you cross the line just as the lights go green. If you and the other car then promptly accelerate at the same rate, I calculate that you will gain five metres on it for every second while you are both accelerating. But before you take advantage of the rolling start, do look left and right for red-light jumpers...

Iíve just noticed, by the way, that this manoeuvre provides an answer to my third question above!

As for the first question: according to the regulations, the meaning of the amber light is Do Not Cross The Line (except when you are too close to it to be able to obey an amber following green). The same goes for red of course. This is also the message in one of the sections in the Highway Code. What surprises me is another section saying that unless green is showing, ďYou MUST stopĒ behind the line ó even though there is really no requirement to stop (until you reach the line).

The second question has a double answer, because the amber light stays on for three seconds on its own but only for two seconds with the red. At least thatís what Iíve observed everywhere, since I started thinking about this monthís column.  Before then I never even noticed the times were different. Call myself a scientist!

Peter Soul

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