Should Carbon Taxes Be Additional to Other Transport Fuel Taxes?

If transport fuel is taxed as a method of charging for road use and congestion, then, as a first approximation, carbon taxes should be superimposed on the existing taxes and the final price of transport fuel should rise by somewhat more than the carbon tax. If transport fuels are already taxed, the cost of meeting the emissions target will depend sensitively on whether the reduction in CO2 emissions is a proportion from base levels or to a target level, depending on factors other than fuel consumption (GNP or population).