Sir – If Nicola Blackwood is really “unhappy with the minister’s answer on A34 investment” (Report, December 12) that should mean her advocating the cheaper and more effective way to increase the capacity and reliability of the road.

The capacity of roads increases with reduced speeds (see the variable speed limits on M25, M42 and M6). Danger, leading to many of the accidents that plague the A34, also increases with differential speeds, eg trucks doing 55mph mixed with cars doing 80mph.

The intelligent response to congestion caused by excessive speed and accidents is to reduce the speed limit of the trunk road along its length to 50mph. It should be unnecessary to limit HGVs to the inside lane when all traffic has been slowed down to its most efficient speed in terms of reducing carbon emissions. There would incidentally be a substantial reduction in both engine and tyre noise (see the length of A34 through North Hinksey).

The only reason I can think of why this has not been done is that this working example would make it even harder for politicians to resist a similar change to the national speed limit.

Such a systemic change would have even more significant benefits including the change to engineering models of car incapable of 120mph and more efficient at 20mph, and modal shift to lower-carbon modes of train and coach. The Treasury has been resisting this change due to the loss of fuel duty arising from more economical (and less polluting driving) but is now having to look elsewhere to replace the tax revenues lost to the Government-sponsored growth of electric cars and reductions in car ownership and mileage.

Daniel Scharf, Drayton