I AM sure that John Macmillan had the best of intentions when he wrote (Letters, May 1) that making vehicles travel at a maximum of 20mph within our cities will dramatically increase air pollution.

This is not the case for the following reasons:

Physical principles: kinetic energy varies as the square of the speed. The square of 30 is 900, the square of 20 is 400. Therefore to accelerate a vehicle to 30mph takes two and a quarter times the energy. Higher energy requirements equate to higher fuel consumption and higher air pollution.

Controlled studies: studies by reputable groups show that travelling at 20mph is more fuel efficient. To quote the Department of Transport when referring to 20mph schemes: "Driving more slowly at a steady pace will save fuel and reduce pollution, unless an unnecessarily low gear is used."

Try it yourself: I made five repeat journeys over the same routes at the same weekday times. Each pair of journeys was identical except that in the first the maximum speed was 20mph, in the second 30mph. When speeds were kept down to 20mph I obtained a 12 per cent average improvement in fuel consumption. I invite Mr MacMillian to conduct his own experiments and write back to The Herald with his results.

All the above show that 20mph speeds give improved fuel consumption which can only be good for our air quality. The ruling from the Supreme Court ("It is long past time to clear the air on urban pollution", The Herald, April 30) makes the need to lower traffic speeds, not only in Glasgow but all our cities, even more urgent.

Bob Downie,

66 Mansewood Road,

Glasgow.