Sir – 20 mph wastes fuel. The DfT says vehicles emit 30 per cent more CO2 and diesels emit 30 per cent more PM10 particulates at 20 than 30. 20 mph increases carbon monoxide and hydrocarbon emissions. Widespread 20 limits can increase climate change and harm public health.

And yet a medical professor, Sir John Grimley-Evans (Letters, March 11), claims it is “socially responsible” for motorists to obey Oxford’s city-wide 20 limit.

Oddly, his “evidence” is a BMJ report that declares “The benefit of … 20 mph zones in reducing road casualties … has not been conclusively established”.

James Styring (Letters, March 18) demands police enforce Oxford’s 20mph limit. Police resources are always finite but especially in a recession, so what other policing does James want cut to enforce 20 mph?

James asks “Must drivers only comply with laws they believe are fair…? No one would apply this weird logic to any other law”. Yes we do: “policing by consent” is how a mere 4,165 Thames Valley Police officers protect over two million residents.

James uses generalised national figures to imply that just because a limit is reduced from 30 to 20, traffic will adopt the lower speed. Actually both the DfT and BMJ say it is intrusive traffic calming, not 20mph limits, that reduces traffic speeds. Because pedestrian survival rates are much higher when hit by a vehicle at 20mph rather than 30mph, James claims city-wide 20mph limits can reduce casualty rates and severity. However, most Oxford residential roads had no casualties to reduce. Oxford already had fewer serious casualties than any rural Oxfordshire district. The 20mph limit wasted £300,000 in Oxford that should have gone to rural casualty reduction.

Indiscriminate 20mph limits harm the environment and without traffic calming they have scant effect on casualties. But they help illiberal, irrational pedal-bullies tell others what to do.

Hugh Jaeger, Oxford