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A PHYSICIST WRITES . . .

(October 2004)

Information ... that vital flow which governs your driving and keeps you safe on the road. Some sorts of information are surprisingly difficult to collect, though. Letís consider speed first, and then acceleration (which to a physicist can mean slowing down as well as speeding up ó but I will try not to confuse you!).

When you are on the move and looking at other vehicles ahead or behind or alongside, you are constantly assessing their speed relative to yours, especially if they are coming steadily closer. You can sense exactly how long you have got before contact (if neither of you take avoiding action, that is).

However, this relative speed is a very different thing from absolute speed, which is the rate at which you are actually moving along the road. I find this sometimes much harder to guess at (before checking the speedometer). On a narrow road I tend to overestimate my true speed, while on a wide one the opposite happens. This is to be expected because the further away the roadside is, the more slowly it will be sliding across my field of view.

But even looking at the speedometer doesnít give you anything like a proper sense of how fast you and other vehicles are travelling. Itís a long time since I had to wait beside the hard shoulder of a motorway but from what I remember, that is the place to begin appreciating what 70+ mph really means.

Somehow the effect is even more alarming if you find yourself watching a deafening M4 from about 50 metres away, as we did recently on a country walk. How can drivers possibly keep their nerve, travelling and overtaking at such a rate?

Back in the comfortable driving-seat of a sound-proofed car on a smooth road, the sensations could hardly be more different. Itís almost as if you are safely stationary and the surroundings are rushing backwards. The relative speeds of nearby cars do not often shout danger. Oncoming traffic simply whizzes past without conveying any idea of its speed at all. Is this, I wonder, the best state of knowledge in which to be driving fast yourself?

[A reader commented that trying to make use of a pedestrian crossing in 30 mph traffic is just as frightening as standing beside the M4 ó but on the other hand, if you are one of the 30 mph drivers then you feel perfectly in control and see very little danger. I think the same two opposing points of view apply to someone walking along any footpath alongside a busy road. Is it the pedestrian or the driver who has got the wrong idea...?]

The only circumstance I can visualize where you make a serious attempt at assessing the absolute speed of another vehicle is when it is crossing or turning in front of you. But even then you may discover you have miscalculated the moment when your path will be clear ó and similarly, it is possible to get the timing wrong when you are overtaking. How does this come about?

The most likely answer is that the other driver has pressed either the accelerator or the brake pedal, without your noticing! The fact is that it can be almost impossible to detect acceleration (either upward or downward) early, unless you are able to hear the engine note or you see the brake lights.

Without these clues you will be in ignorance, until you suddenly realize that the other car is not in the position you expected it to be in. Perhaps this hazard is an obvious one, but I donít find always remember to allow for it when approaching a hazard or planning a manoeuvre.

Incidentally, now that many cars possess a third (central) brake light, why havenít manufacturers thought of wiring it separately to indicate heavy braking? This would be easy enough to do, and would be much more informative than the present signal which tells you that the car ahead is braking a bit, or maybe a lot (or maybe even not at all).

Thereís one situation where you can instantly detect acceleration just by looking at the wheels, namely when the other vehicle is stationary. I always watch the front wheel of a waiting bus or car, for the first sign of movement and indeed for advance warning of the direction it will be in.

Recently, however, I was astonished to see a car waiting to emerge from a petrol station but with the wheels apparently already turning. Then I realized I was looking at a set of exotic free-wheeling hub-caps! They eventually stopped spinning and then stayed that way as the car moved off. I found this sight extremely disorientating, and I am hopeful that hub-cap bearings soon filled up with road dust.

I havenít even touched on the other vital aspect of assessing speed and acceleration: the fact that drivers around you are attempting to assess yours. Another time, perhaps.

Peter Soul

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