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Two Fallacies Exploded
Page 62
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1\4 UCH arrant nonsense has been talked both inside and outside Parliament about the dangers of allowing heavy vehicles to travel at unlimited speeds on motorways. There is a widespread but fallacious belief that, because of their weight, large lorries are incapable of stopping within a short distance. Nothing could be further from the truth.
A well-known make of car, capable of nearly 100 m.p.h. and equipped with the latest type of brake, requires 51 ft. in which to stop from 30 m.p.h. A representative eight-wheeler, laden to 24 tons gross, needs only 49 ft. These facts have been established in tests conducted by The Commercial Motor. Using a decelerometer, however, the car shows 100 per cent. braking efficiency, equal to a stopping distance of 30 ft. from 30 m.p.h.
The decelerometer is a deceptive instrument for measuring braking efficiency. The only reliable device is the gun, as used by The Commercial Motor, which fires a chalk pellet on to the road at the first moment of contact between the foot and the brake pedal, and enables the true stopping distance, including delay in the system, to be measured with a tape. A high-efficiency reading can be obtained with a decelerometer on an ineffective braking system merely by jabbing the brake pedal fiercely and locking all wheels. Opponents of high-speed operation of heavy vehicles also overlook their inherent stability. Thii certainly does not apply to a number of cars, which, apart from rolling and pitching, are too light to be able to stop in anything approaching the theoretical minimum distance. If their true braking efficiency were checked with a gun, many users—and, perhaps, manufacturers—might be surprised.
If there is a case for imposing a discriminatory speed limit on any type of vehicle on motorways, it lies against the private car. Well-maintained modern commercial vehicles are far safer in an emergency.