Sir — Cycling is my favourite way around Oxford. I exercise, enjoy the birdsong and leave almost no carbon footprint. Our county council wants to encourage walking and cycling in Oxford. It claims 20mph limits would do this. But 20mph limits can also increase pollution. Do city-wide 20mph limits reduce motoring enough to avoid increasing emissions and climate change?

Cars emit 30 per cent more CO2 at a steady 20mph than at a steady 30mph. Diesel cars emit 30 per cent more PM10 particulates at 20mph. Road humps reduce engine efficiency: increasing carbon monoxide emissions by 82 per cent, nitrous oxides by 37 per cent, emitting more hydrocarbons and wasting fuel.

Oxford’s cycleways are not a network, most are narrow, and only a paint stripe divides them from pedestrians or traffic. In Graz, police enforce 30 km/h zones. Thames Valley Police would not normally do so.

20mph zones in Hull have greatly reduced casualties, but they focus on hazardous areas that form only 20 per cent of Hull’s streets. In most Oxford residential side roads, traffic varies between 15 and 25mph and there are already few or no casualties: 20mph limits would make scant difference.

Oxford’s casualties are mostly on main roads. About half are on roads that Oxfordshire would leave at 30mph. On main roads that would become 20mph, Oxfordshire has ruled out traffic calming. 20mph limits without traffic calming reduce average speeds by 1mph, which on main roads reduces casualties by only four per cent.

Many urban road casualties involve vehicles breaking 30mph limits. Would casualty numbers and severity be reduced more by road modifications and enforcement to keep traffic within 30mph, better cycleways and wider pavements?

Hugh Jaeger, Oxford