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A PHYSICIST WRITES . . .
(May 2004)
Here is a mixed bag of motoring oddities:
The priority rule at roundabouts is that you should give way to traffic coming from the right. How is it, then, that when you arrive at a busy 3-way mini-roundabout with all the traffic signalling right, the flow is usually smoother if everyone lets the vehicle on the left go first and then crosses immediately behind it?
Answer: this works because you can usually see that the vehicle on your right will be blocked by the first one (from the left) long enough for you to proceed safely through. If everyone stuck to the priority rule, the flow-rate would probably be halved.
This left-first flow at busy times is a remarkable example of rule-breaking by consensus. When there is less traffic, other curious things can happen. Suppose I am approaching the roundabout and two vehicles are already waiting there, each “giving way to the right.” If I stop too, there is likely to be a stalemate.
If instead I continue cautiously through and to the right, I feel I am taking unfair advantage of arriving last and being on the move. Very often, though, doing this enables the left-first flow to start up behind me. Did I read somewhere that mini-roundabouts are deliberately designed to seem dangerous, so that drivers will apply some caution?
A question about relative speeds: when you’re on the M4 near Heathrow (as a passenger rather than a driver, I should add) why do landing aircraft seem to hang almost motionless in the air — whether they are travelling in your direction or the opposite one?
Answer: if it’s the same direction, then even though a plane may be moving 100 mph faster than you are, this difference looks quite small a mile or two away (and with the scenery rushing past). When the plane is landing in the opposite direction to you there is often something on the skyline — closer to you than to the runway — which stays in line with the plane, thus giving the impression that it is hovering.
And if you drive west on the M4 past Reading, have you noticed that after the Tilehurst water tower first appears on the right, on the distant skyline, it seems to grow magically in size? The reason for this is that the motorway curves very gently to the right for about two miles. Along this stretch, therefore, the tower holds its direction relative to the car, and so you get the impression that it is further away than it really is.
But in fact over the two miles your distance from the tower is halved — hence it appears to double in width, but unexpectedly. If it didn’t become mostly obscured by trees as you approach, the effect would be even more startling, because the apparent size of its visible area would increase by a factor of four.
Finally, any road sign that shows a road layout, for example a staggered cross-roads or a roundabout, is obviously meant to be ‘read upwards’ as if you are traversing it from bottom to top. Why then is a pair of signs on a single column always arranged so that the lower hazard is the one you will meet second? I can’t find any mention of this illogical rule in the book Know your traffic signs.
Except, believe it or not, I’ve just noticed that a new pair of signs around the corner from home is arranged in the wrong order (ie, the logical way)! I am torn between reporting this error to the local authority and putting up some barbed wire so that they can’t spoil the logic.
Peter Soul
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