Sir – Ted Dewan and Lady Grimley-Evans (Letters, January 1) dismiss Paul Wooldridge’s questions with blithe assertions and disregard NL Gregory’s fears about cars doing 20mph (December 11). Back in October, I cited DfT data that suggests an Oxford-wide 20 mph limit would scarcely reduce casualties and could worsen emissions.

Oxford’s 20mph lobby has yet to offer factual evidence to the contrary.

Dewan and Grimley-Evans’ claim of “smoother traffic flows” cannot include 20mph limits with humps.

In Walton Street, humps make drivers vary repeatedly between ten and 20mph. If I cycle there, I frequently have to brake for vehicles slowing in front of me, and am then stuck behind them breathing their exhaust at the same moment that their deceleration sharply increases their carbon monoxide and NOx emissions.

Oxfordshire’s Casualty Report 2006 stated that in Oxford “the risks to individual cyclists appear to be appreciably lower than the national average”.

Despite this, another 20mph advocate wrote to you misleadingly claiming “According to county council data, Oxford city pedestrians and cyclists are three times more likely to be involved in an accident than residents of neighbouring council areas”.

Only on roads with significant casualty rates can 20mph limits’ benefits outweigh their environmental harm. Installing them more widely diverts Oxfordshire’s safety budget away from roads that really need casualty reduction measures.

Oxfordshire’s highest casualty rates are on rural roads.

Councillor Hudspeth admits that casualty reduction is not the purpose of an Oxford-wide 20mph limit. In most of Oxford’s residential side roads, traffic speeds are 15-25mph and casualty rates nearly zero.

Most of Oxford’s casualties are on its main roads. The DfT’s New Directions in Speed Management shows that many urban casualties involve vehicles breaking 30mph limits. Oxford needs 30mph enforcement, not 20mph gimmickry.

Hugh Jaeger, Oxford