Sir - If the whole of Oxford is made 20mph zone(Report, June 13) the county council should look at the evidence relating to carbon emissions. Current vehicle technology is inefficient at lower speeds, particularly if lower gears are used to avoid exceeding the speed limit.

Alternatively, carbon emissions might be reduced if the lower speed maintains smoother traffic flow and reduces congestion by increasing the capacity of both roads and junctions.

There would be further savings if safer road conditions increased walking and cycling. However, the substantial reductions that are required to be made in carbon from transport will not be achieved until the speed limits between towns are also reduced.

For example, 50mph was recommended by the UCL/Halcrow VIBAT study commissioned by the Department of Transport in 2005, 55mph by GreenSpeed since 1988 and 60mph by UK Energy Research Council in 2007). While a paltry and inadequate saving of only six per cent by 2013 is expected by the Manchester authorities from its package of measures, including peak-hour congestion charging.

Savings of 60 per cent would be possible if the county council reduced speed limits on all county roads and trunk roads (as has been achieved with the Highways Agency on the Western Bypass/A34).

Lower speeds not only reduce noise, improve air quality, and increase the attraction of public transport, but with national application will trigger a shift in technology such that even 20mph would be an efficient speed.

In 1973, when oil supply was not dissimilar to that in 2008 and which we can expect to prevail for the foreseeable future, the Government reduced the speed limit to 50mph.

Daniel Scharf, Abingdon