For most Londoners, this won't make a difference

But will Ken Livingstone be there to witness this as Mayor? Today's announcement is one of his clearest "class war" policies in his eight years at City Hall. Whether it wins him a third term is another thing.

The surprise about today's decision is the lack of surprise. It appears that weeks of deliberations have led to little change in his thinking. It departs little from his basic intentions announced last year.

Initially Mr Livingstone had hoped to scrap the £8 weekday charge for vehicle with lowest emissions before May's election. Those drivers will now have to wait until October for their licence to roam. As for owners of gas guzzlers, they now have one less reason to vote for Ken. They probably weren't going to anyway. But will they back Boris instead? They probably were going to anyway. It's just more likely that more people will actually force themselves to the polls on 1 May.

Mr Livingstone's Tory rival has already said he would scrap the western congestion zone extensions. That would remove at a stroke the discriminatory nature of today's changes - the fact that the 90 per cent residents' discount will be removed for drivers of the most polluting cars. But remember that reverting back to the central zone would remove the opportunity for west Londoners to drive around for 80p a day.

The changes will also alter the scheme's delicate finances. Ken relies on the £120 non-payment penalty to ensure the congestion charge washes its face financially. But if you don't have to pay £8, you're never at risk of paying £120. The occasional £25 fee is unlikely to rebalance the books.

As in the last mayoral election in 2004, when Mr Livingstone proposed extending the zone into west London and then Tory candidate Steve Norris proposed abolition, voters will have a clear choice. Expect lots of headlines and hot air. But it's not the issue that will decide the election.

For most Londoners, it won't make a blind bit of difference to their daily lives. If they travel into central London during the week they use the Tube, train or bus. If they need to use their car, they'll still have to pay £8. A canny few - probably Chelsea residents - will switch to exempt small cars, and whiz around unfettered.

If Ken really wanted to go green, he would have tightened the rules on exemptions, not relaxed them. Congestion in central London is going to get worse, not better. That's the real issue come 1 May.