In your editorial you suggest that a graded toll could be introduced to reflect the type of car. But would it also reflect how well the engine was tuned, whether it had suitable tyres, what type of fuel it was using, whether the driver was driving sensibly, whether it was carrying a rack or towing a trailer, or any other factors which would affect the amount of pollution generated?What we need is a planned increase in fuel tax, with the money being used to create a more effective public transport system. Cars are only part of the transport system, and we need less polluting alternatives. Congestion charging certainly has a role in many localities, but it should never be an excuse to stop taxing pollution.MARTIN JUCKESOXFORDSir: There is an even greater problem than traffic congestion and that is its consequential emissions of carbon dioxide which are steadily rising.

It is this which should be targeted.One way to do this would be to provide every road user with a "ration" of carbon dioxide. The allowance could be embedded in a smart card which deducts the carbon content each time fuel is purchased. The card would be issued with the annual or half-yearly tax disc. If the allowance was exhausted there would be a punitive tax added to each subsequent purchase of fuel.

Alternatively carbon credits could be purchased from road users with a surplus of credits via a carbon trading scheme. At the same time it would provide a strong incentive to switch to more carbon efficient vehicles.In each case the price would be set by government and adjusted to reflect the nation's performance in meeting its carbon dioxide abatement targets. Obviously there would need to be fine tuning to redress the imbalance between rural and urban residents and to allow for the disabled and pensioners. But this would be far more cost-effectivethan journey pricing based on satellite technology and would remove any suspicion that the prime purpose is to raise revenue.PROFESSOR PETER F SMITHUNIVERSITY OF NOTTINGHAMSir: With the Government considering monitoring the position of vehicles on the road, ostensibly to enable charging for road usage, yet another means of controlling the population becomes a possibility. We are already observed by a multitude of CCTV cameras and recorders and the location of an active mobile phone can be readily monitored. With further technological development it may soon be possible to have the proposed ID cards tracked continually, thus enabling the government, or other agencies, to keep tabs on us.Is this creeping erosion of civil liberties what the people of this country really want, or are we just sleep-walking into a slightly delayed 1984?KEITH BARNESWHITWELL, HERTFORSHIRESir: I was appalled when the socialist mayor of London, Ken Livingstone, introduced the congestion charge which clearly discriminates against those on lower incomes.