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A PHYSICIST WRITES . . .
(October 2012)
Our range of speed limits here in the UK, already quite wide, is being further stretched. At the upper end, a year ago the government announced a public consultation on raising the motorway limit from 70 to 80 mph. The IAM immediately responded with a call for a full risk-assessment, in the form of a pilot study on one of the motorways that have a variable speed limit (such as the M25 in Surrey).
Other organizations and experts later expressed a variety of opinions: if an 80 limit is strictly enforced, speeds will not rise significantly, and nor will accident rates ... but there will be increases in the occurrence of asthma (because of raised air pollution) and of obesity (“due to more people taking advantage of shorter car journeys”!) ... and if, as some predict, there’s a 3% rise in average motorway speeds, the result will be an extra 25 deaths and 100 serious injuries a year ... also, after leaving the motorway drivers will adopt higher speeds than at present on adjacent roads.
Damon Hill said in an interview: “Most people aren’t safe to drive over 55,” (meaning speed rather than age). He thought the limit everywhere should be 55 mph or less, as in the US. Well, you can sympathize with someone coming off the racing track and then being frightened at the lower driving skills being displayed around them on the open road. Me, I am always astonished at how expertly most people drive, at least as far as managing to avoid each other is concerned.
But it also surprises me that drivers seem largely unaware of how rapidly fuel consumption rises with speed, above about 60 mph. This is partly because you are moving out of the optimum operating range of the engine, and partly because of air resistance. The latter goes up in proportion to the square of the speed, so at 80 mph the drag on the vehicle is (roughly) 30% higher than at 70. And because of the higher speed of pushing against the higher drag, the engine is actually having to deliver 50% more power!
The engine efficiency shouldn’t drop a great deal, from one speed up to the other, but the combined effect of this and the drag is that consumption (in terms of litres per mile) can rise by up to around 20%. Advanced motorists are expected to “make good progress” by driving near to the speed limit (when it’s safe to do so) – but how can this aim be consistent, at 80 mph, with the economical habits that we are also supposed to follow?
There’s yet another aspect to the higher limit: the Road Safety Foundation has inspected the motorway network, and reports that much of it does not have adequate crash barriers and information-display systems for speeds of 80 mph. I’ve read too that general motorway design standards (eg, concerning lane width and bend radius) have been relaxed everywhere over the years, for reasons of space and cost restraints, with the Department of Transport giving approval for this specifically on the basis of the 70 limit. It seems therefore that raising the limit to 80 mph will require an upgrade to the whole network. The logic of this can’t be avoided even by those who claim that actual speed levels won’t increase (and it applies even if the speed limit itself isn’t changed, I would say).
Meanwhile, at the other end of the speed range there are regular calls for more 20 mph zones to be established, and even for all built-up areas to have a 20 limit. The usual justification given is that people (pedestrians or cyclists) hit by vehicles at 20 are more likely than not to live, whereas those hit at 30 are more likely to die.
This may be true – but I can’t help reacting by suggesting that it would be better if driving (and cycling) standards were forcibly improved, to the point where everyone adopted an appropriate speed (and conspicuousness, in the case of cyclists) for the conditions, and no-one at all was hit. Why, in this utopia of mine, speed limits would be unnecessary and we could give full attention to the road instead of having to keep an eye on the speedometer...
Returning to the real world of accidents, let me offer a possibly insensitive thought: raising the 70 limit to 80 is likely to generate headlines that refer to “additional lives lost”, just as extending the 20 mph limit would result in “some lives saved”. But in reality, both on motorways and in built-up areas, a change in speed limit will mean that nearly all vehicles are in positions (relative to each other) that are different from what they would have been, and hence nearly all collisions will involve different sets of people. Thus, after the change, anyone involved in an accident could reasonably complain that if only the limit had been left as it was, they would have arrived at their destination unscathed.
To me, this is another illustration of the ‘chaotic’ nature of road travel, which I tried to explain in my September 2011 column: the point is that even a tiny change in someone’s position or speed can ripple out and affect the journeys of many others. But of course you can never know what might have happened on your journey under even slightly changed circumstances, so the best thing is simply to concentrate on driving carefully!
Last May I repeated my warning that in any country where the regulations require you to carry spare glasses when driving, one pair will not be enough, because if you should need to put them on in an emergency, you will immediately be breaking the law. A reader pointed out that a similar doubling-up is necessary with spare bulbs, and indeed with any compulsory accessory that you might need to fit before driving on.
And someone asked me about the popping sound you sometimes hear when driving at high speed with a window down: is this the Doppler effect? I said no (for a description of that, see September 2002) – the air passing by is simply causing regular vibrations of the air inside the car. What you’ve got, in effect, is a poorly designed orchestral bass-flute on wheels!
Peter Soul
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