Sir – I fear that Hugh Jaeger (March 25) may be out of his depth. It is conventional for a report of a scientific study to start with an introduction stating why the study was done.

The authors of the BMJ paper explained that they did their work because they felt that more evidence was needed on the effectiveness of 20mph speed zones.

It is from this background section of their paper that Mr Jaeger plucked his misleading quote. Had he read the report further he could have realized that the whole purpose of the study was to plug that perceived evidence gap.

The authors did so with a rigorous analysis of a large body of new data. Their (peer-reviewed) conclusion that “20mph zones are effective measures for reducing road injuries and deaths” is clear enough. Mr McArthur-Christie (Letters, March 25) questions whether speed limits without associated traffic calming measures will achieve anything.

Certainly, a much-cited 1998 study reported that 20mph speed limits with traffic calming measures reduced average car speeds more than did 20mph limits alone.

But one can expect that the difference would now be smaller where more car drivers respect the right of other road users and residents to a quieter and less challenging urban environment. It is basically a matter of good manners.

Wide-area traffic calming in Oxford would be immensely costly, impracticable, and environmentally damaging.

We would not want to re-engineer Oxford as if it were Slough, and cannot re-design it as if it were Copenhagen.

County policy-makers responded sensibly to the available evidence in introducing the speed limit, but it is up to us to seize the opportunity for making Oxford a pleasanter place. At the very least, if enough of us drive at 20mph in Oxford’s narrower streets other people will have to!

John Grimley Evans, Green Templeton College, Oxford