Sir – Although the results of your experiment (New 20mph limits ‘unsafe’, May 14) are not statistically signifcant, there is a more fundamental reason why this test of the effect of slower speeds on driving in urban areas is misleading and unhelpful.

The transport system that needs to work towards a reduction of overall carbon emissions by 80 per cent to 90 per cent includes many other modes of travel, and the use of vehicles both within and between urban areas. In terms of emissions, it is the 70mph limit that needs urgent attention.

In 2006, the UK Energy Research Council recommended a reduction to an enforced 60mph as a ‘Quick Hit’, saving almost 2MtC of carbon annually.

Slower car speeds (and 50mph has also been recommended to the Department for Transport as an essential part of low carbon transport) would trigger a virtuous circle within the transport system as a whole, reducing carbon still further through both greater use of public transport, walking and cycling and a reduction in congestion.

There would be a shift from vehicles capable of over 100 mph as used in your experiment, to vehicles that would be more efficient at slower speeds (including 20mph).

With a full adoption of 20mph throughout urban areas the consequent increase in road and junction capacity might avoid any increase in carbon emissions even with the current vehicle fleet, but your report is useful as it draws attention to the fact that this might not be the case, and that it is the reduction of the 70mph limit which is both urgent and necessary.

Daniel Scharf, For GreenSpeed, Abingdon