London mayor Ken Livingston has announced plans for a differentiated congestion charge in central and western London, favouring vehicles with low CO2 emissions and punishing high-polluting cars like the gas-guzzling four-wheel drives known as Chelsea tractors. Under the proposed scheme, which would come into force in 2008, cars with low emissions would pay less than the current 8 rate, while charges for environmentally-damaging cars could rise to 25, to be further increased in 2010. Livingston has described the proposal as a charter for greening our car choices, putting London at the cutting edge of both environmental and transport policy.