Sir – Mark Lynas’ books and journalism emphasise the scale of threat from man-made climate change, the extent that humankind must reduce its emissions, and that we may have only a few years before it will be too late.

It is, therefore, perverse that he demands motorists in Oxford be forced to increase their CO2 emissions by 30 per cent and diesel drivers to increase their PM10 particulate emissions by the same amount.

The DfT tells us that these and other emission increases are the effects of traffic running at 20 rather than 30mph.

Supporters claim lower limits will reduce emissions because traffic will spend less time accelerating up to the maximum permitted speed and then slowing down again, both of which are less fuel-efficient than constant-speed motoring.

But our county council did no traffic modelling to see whether reduced acceleration and deceleration would offset the engine inefficiency of driving for longer at 20mph in third gear.

The 20mph supporters’ claims are, therefore, speculative.

Before indiscriminate 20mph limits were imposed, supporters blithely dismissed warnings that speed limits must be “self-enforcing” as police would not normally divert resources to enforce them.

Mark McArthur-Christie (Letters, September 3) rightly predicted that after the signs went up, motorists would disregard them and then frustrated supporters would demand enforcement.

Unless police resources increase, enforcement will be by installing more hazardous and ugly traffic “calming” obstacles, markings and signs. Repeated slowing and speeding up for these will reduce engine efficiency even more, making emissions even worse.

Installing and maintaining traffic calming will divert resources away from what pedestrians and cyclists need more: to have pavements and road surfaces fixed properly.

What even Mr McArthur-Christie did not predict was Mr Lynas equating people who exceed a 20mph limit with paedophiles. Such inflammatory and disproportionate language discredits this debate.

Hugh Jaeger, Oxford