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A PHYSICIST WRITES . . .
Here are some afterthoughts to September’s column on vehicle separation and stopping distances. You may have seen University Challenge soon afterwards on BBC2, in which the students were shown a pair of Thinking and Braking Distances from the table in the Highway Code and were asked what speed they corresponded to. The guesses from each team were hopelessly low! But then a further three pairs of distances were answered almost immediately with the correct speeds. I suspect that one bright young brain realized that the speed would be proportional to the Thinking Distance...
My general conclusion in September was that it’s better to learn the look and ‘feel’ of the distance ahead you can safely stop in — rather than memorizing the table in the Code — and I am sure most drivers do this anyway. But are you aware of some particular situations (apart from bad weather!) when you ought to allow considerably more than a normal stopping distance ahead? I’m thinking, for example, of when you are being tailgated or at least closely followed by one or more vehicles. If you had to do an emergency stop, there would be no chance of avoiding a shunt plus the argument over whose fault it was.
Or suppose you take your eyes off the road for a second (even for a good reason such as to check your speedo or mirrors). In one second at 70 mph you will travel one-third of the Typical Stopping Distance of 96 metres. So before looking away, you really ought to be sure that a similar further length of road is hazard-free.
Here’s another situation you might not have thought of: on a winding single-track road you should allow at least double the normal stopping distance. Why? Because an oncoming vehicle, suddenly appearing, will need the same distance as you to stop in — or even more, if it’s moving faster! Paradoxically, of course, such roads become much safer after dark when headlights give you advance warning of approach.
I’ve also been investigating the stretches of chevrons you may have encountered, painted on about half-a-dozen isolated stretches of motorway around the country and accompanied by signs saying: “Keep apart 2 chevrons.” What does this instruction mean exactly, and how far apart are the chevrons themselves? I sent an enquiry to the Dept for Transport.
I was told that the chevrons have a 40 metre spacing and that drivers should ensure they can always see two chevrons between them and the vehicle ahead. Then allowing for 10 metres of road being obscured by the front of your car, you will be keeping to a 90 metre separation which is close to the Typical Stopping Distance at 70 mph.
But is this how all drivers interpret the instruction on the signs? I doubt it! Some probably aim to have two chevrons in view just part of the time (as they pass into and out of sight) which means that the separation could be as low as 50 metres. I would have thought “Always have two chevrons in view” would be a clearer instruction. Though even this would not provide a safe stopping distance between vehicles travelling at more than 70 mph.
I then asked why it was that I had seen chevrons on the M1 painted only in the inner and middle lanes — was it perhaps because drivers never travel as slow as 70 in the outer lane? No, came the answer: the M1 chevrons were the first ones to be laid, copying a trial in France which didn’t include the outer lane. And this was probably because the lane was regarded then as purely for overtaking rather than tailgating. How pleasant les autoroutes must have been to drive on in those days (I’m told they still are)!
Undoubtedly, some drivers on our motorways ignore the chevrons altogether, but it’s known that they do result in a distinct drop in the amount of tailgating, not only where the chevrons are located but also for some further distance along the motorway. So why have they not been applied much more widely? I believe the reasoning is that motorists would stop reacting to them, becoming too accustomed to seeing them. It’s just as well this theory doesn’t apply to traffic lights...
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