Sir — I must thank Bill Leonard for his contribution to the debate about the effect of lower driving speeds and speed limits (Letters, May 28).

Unlike him, I make no claims to know much about climate science, but that does not stop me from being very concerned about methane escaping from melting permafrost, the albedo effect of reduced reflective ice and snow in the Arctic, the threat to both rain forests and North American pine forests, the reduced carbon absorption of the oceans, shrinking of glaciers in the Alps and Himalyas and the melting of ice in the Antarctic and Greenland; all part of environmental feedback loops and all attributable, to some extent, to increased concentrations of atmospheric carbon.

As for his suggestion that GreenSpeeders would not own cars, membership is actually signified by a 55>20> bumper sticker that signifies to the driver behind that a car is being driven at a socially (reduced injury and loss of life), economically (lower cost of fuel and insurance) and environmentally (reduced emissions and congestion) responsible speed.

If Bill Leonard knew as much about the transport system as he purports to know about the climate, he would be aware that lower speeds, if observed by all drivers, increase the capacity of roads and junctions (see the variable speed limiting on congested parts of the motorway system).

With cars designed for the lower speeds that are already the legal maxima for goods vehicles, drowsiness and lack of concentration would not be a problem.

Hence the urgent need is for systemic change; a lower and properly enforced national speed limit, rather than just more GreenSpeed bumper stickers or 20mph zones.

Daniel Scharf, GreenSpeed