Sir – The argument at national and local levels about the merits of slower speeds in urban areas seems to tipping in favour of 20mph in order to reduce the number and severity of accidents.

However, we should not lose sight of the other critical impact of the motor car, carbon emissions. 20mph is an inefficient speed for the current generation of overweight and overpowered cars and carbon emissions will increase unless congestion is reduced (as it might be) and/or there is a modal shift to walking, bus and bike (that is less likely).

Without disparaging the benefits of 20mph limits, looking at the transport system as a whole, there would be even greater benefit in reducing the national speed limit to 50mph.

This would trigger a shift to lighter and less powerful cars that would be both more efficient and quieter at 20mph, less prone to breaking that limit, and more conducive to the use of alternative fuels.

There would be more medium and long distance journeys where the coach (being allowed to travel at 65mph or more on the overtaking lanes on trunk roads and motorways) or the train (without the need for a higher speed rail network) would become the more relatively attractive options.

The Department for Transport is is prepared to admit that it is aware of these and other advantages of a lower national speed limit (such as the reliability of the transport system as a whole), but is waiting for the need to save carbon to to be absorbed into the public consciousness before introducing what is currently considered to be 'politically impossible'.

Daniel Scharf, Abingdon