C-Charge chopping and changing

I was interested to see a brief report in yesterday's Times suggesting that Ken Livingstone was about to drop the Congestion Charge exemption he had planned to introduce next year for cars with official CO2 emissions figures of 120g/km or less.

Apparently, so many cars are now sneaking in under the limit that Ken is worried they will flood the streets of central London. I can see why he feels he has to make the change; after all, I don't suppose the BMW 318d was the sort of hair-shirted economy car he had in mind as benefitting from the exemption when he first suggested it. But on the other hand, the prospect of avoiding the C-Charge has obviously provided a useful spur to the car manufacturers to improve their emissions performance; they and their customers are probably entitled to feel cheated now that the proposed concessions won't be available. The real problem is that having been burnt once, the car-makers might not respond to incentives and initiatives of this sort in the future for fear of being messed about again.