It isn't often that I take the time to praise Transport for London, but here goes. (I offer the usual apologies for being London-centric. It's just that it's about to widen the Congestion Charge zone, and other plans to change the scheme radically are gathering pace. One day every city will have a scheme, so if you live in Leicester or Liverpool, it's worth noting what they're up to in the capital.)

The startling news is that the Transport for London press team were able to explain their plans to me, apparently honestly and certainly courteously, and without a hint of the paranoia that normally haunts a quasi-political organisation such as TfL. I wanted to know, on my own behalf and on behalf of thousands of others, how the new Congestion Charge scheme would affect owners of cars that are now exempt from the charge but will, before too long, become liable.

TfL wants to change the way the charge is structured from the current, slightly anomalous, rules to a scale based on carbon dioxide emissions, and in line with the current government bands for vehicle excise duty.

I was greatly relieved to learn that, on the present proposals, cars such as the Toyota Prius and Honda Civic Hybrid, the two medium-sized cars that currently have a zero-charge rating, can continue to travel in and out of central London for free. Indeed, all Band A and Band B vehicles will receive that privilege. So, instead of being restricted to an eccentric collection of hybrids and electric cars, buyers now have a wider choice of more conventional cars; provided they emit less than 120 grams of CO2 per kilometre travelled, they can go into the zone for nothing. Thus drivers of the Smart car will now be exempt - and so will drivers of the least-polluting diesel Citroën C1s, C2s and C3s, Fiat Pandas, Ford Fiestas, Toyota Aygos and Yarises, Peugeot 107s and 207s, Renault Clios and more.

This is all very good news, even good enough to balance the £25 per day charge that Mayor Ken Livingstone apparently wants to see slapped on the more polluting cars in Band G - those emitting more than 225g/km, and hitting limos, out-and-out performance cars and SUVs. You see, if you can afford to buy a Lamborghini or a BMW X5, surely you can find a few quid to buy a Smart as well, thereby ensuring that you have the best of both worlds. Seems fair enough to me.

The only problematic area concerns models that are currently exempt from the charge, and whose owners bought them before the proposed new range of tariffs was published but who, on the face of it, will be stung when the new regime kicks in.

There are comparatively few of these, in three categories: first, the big Lexus hybrids that qualified for exemption because of their technology rather than the absolute level of CO2 emissions; second, cars converted to run on LPG; and third, older vehicles, including classics. A 1959 Mini, say, is still cleaner than a new Range Rover and it shouldn't be charged in the same way. Nor should owners be hit "retrospectively" - quite an important principle.

Otherwise, congratulations Transport for London for getting most of it right.

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