Changes put clear water between Mayor and Boris

The most controversial aspect is that the "gas-guzzlers" will no longer be entitled to the 90 per cent residents discount: instead, their owners will get charged every time they drive anywhere within the zone. Just turning the ignition on and going round the corner will cost them £125 a week. The motoring organisations are predictably squealing about how unfair the world is to motorists.

Don't be fooled: we have been here before. After five years, a balanced assessment would suggest that the charge has been modestly successful in reducing congestion. It is a rather blunt instrument but, by and large, it has been successful and while not exactly popular, there have been no poll-tax type riots.

Nor will there be. The measures may appear controversial, but in reality they are pretty modest. Most Londoners will be unaffected.

The decision to allow smaller cars to enter the zone for free is the strangest part of this package. After all, even the most modest vehicle contributes to pollution and if many Londoners trade in larger cars to obtain free entry, the purpose of the charge could be lost.

Mr Livingstone and his advisers are taking a punt on this not happening. Small cars are a minority of those entering the zone - and not many motorists would trade in their pride and joy for the sake of saving the odd £8.

The riskiest aspect of the proposals is that they will not be implemented before the election. They will elicit the predictable furore before anyone knows if they work out in practice.

Mr Livingstone's plans are worth two cheers. He is right to view congestion as a wider environmental problem. Larger cars place an unnecessary burden on London's roads, taking up excessive space; it is amusing that the old Lefty Livingstone is using market forces to allocate scarce road space more sensibly.

We should be grateful to him for putting forward a controversial policy that will raise the level of debate above the antics of the Ken and Boris show. His ideas put clear blue water between the two parties.

This is where Ken the canny old campaigner could wrongfoot Johnson, who has reacted uncertainly to the low-emission zone. Livingstone knows that most Londoners never drive into the congestion zone during the week and very few own gas-guzzlers.

Make no mistake. This is a political move to help the Mayor back into City Hall. If Mr Johnson goes all out to attack the changes, he could find he is out on a limb. Mr Livingstone, by contrast, isn't taking a risk: if the election turns into a referendum on the congestion charge, there can be only one winner. Christian Wolmar is the author of Fire & Steam, A New History Of The Railways In Britain (Atlantic Books).