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20mph speed limits are NOT cleaner

Dear Spurtle

Re your piece on 27 August ('Council consults on slower city'), one misconception that you have fallen into: slower traffic may be quieter, certainly is safer, but is definitely NOT 'cleaner'.

This is because at 20 mph most cars have to be in third gear, while at 30 they can be in fourth, or even on straight runs like Queen Street in fifth. A little rough estimating on my car suggests that I can go between a third and a half further distance for the same fuel at 30 in fourth gear than at 20 in third. In fifth I may nearly go twice the distance. Speed bumps and narrowing with give ways mean more second and even first gear driving, so the problem becomes worse.

So, although I welcome enforced 20 zones in back streets, extending them to main roads is much more problematic, and any speed reduction measures will inevitably make the already too high city pollution levels even worse.

Michael Anderson

(Broughton Place)