Sir – In his letter (March 11), Professor Sir John Grimley Evans confuses the effect of 20mph limits alone with that of 20mph limits enforced with traffic calming.

He quotes a study claiming crash reductions for 20mph zones enforced with calming; but the Oxford scheme was intended to be self-enforcing without the use of speed bumps and chicanes. I have been concerned throughout the consultation and implementation processes that this is a major flaw.

Expectations have been raised that 20mph limits will reduce speeds on roads where, because of their alignment and open views, 30mph is perfectly safe.

The limits will not deliver on these roads — drivers drive to road alignment and characteristics, not limits. On other roads, because of parked cars and narrow alignments, most drivers are already close to 20mph — so there is no need for the limits. A double fault.

These unfulfilled expectations will lead to massive drains on council funds as residents clamour for calming to enforce the new limits.

In turn, conventional, conflict-based calming will increase noise and air pollution and the associated road furniture will disfigure many residential areas.

This is exactly why I have argued throughout for shared space schemes as a more effective, long-term way to reduce speeds in urban areas.

By reshaping the roadspace, these schemes emphasise that residential areas are for people, not just cars. They use ambiguity, rather than conflict, to reduce speeds. They also — unlike calming — add greatly to character and attractiveness of residential areas.

Of course, shared space schemes, whilst effective, do not give the gratifying sensation that drivers are somehow being punished. Perhaps that’s why some organisations in the city prefer less effective and much more unpleasant, conventional calming or camera-enforced limits.

Mark McArthur-Christie, Chairman, Oxford Area Group of Advanced Motorists, Bampton