Sir: Nicholas Schoon quotes Michael Meacher as saying that there must be "more discriminatory use of cars" ("Prescott's global mission", 19 November). This is a classic example of a worthy aim to which all will subscribe and from which most will find clever grounds to claim exemption. So how can we be cured of such a deeply ingrained addiction?

The quick answer is by road tolls and an increase in the price of petrol and diesel. However, there is good evidence that most car users will tolerate substantial increases in cost. The real victims will be small businesses, the poor and nearly poor.

Why not focus on carbon? An alternative to the blunt instrument of pricing could be the carbon smart card, a credit card charged with a given number of kilograms of carbon as an annual allowance. Each time a fuel was bought, the card would be debited the appropriate number of units according to the carbon-intensity of the fuel.

The allowance would need to take account of people with special needs and those in rural locations. It could be manipulated year by year to reflect the country's performance in meeting CO2 targets.

The scheme is not discriminatory or inflationary and would provide an incentive to manufacturers to accelerate the production of ultra-efficient and hybrid vehicles. It would achieve public acceptance since it is targeted at a global rather than a domestic problem and is not a thinly disguised tax.

Professor PETER F SMITH

Royal Institute of British Architects

London W1