It has been suggested that nestling amid Gordon Brown's 10th Budget on Wednesday will be an increase in taxes for the large four-wheel-drive vehicles that are an ever more common sight on our roads. We are told that there will be a new top rate of vehicle excise duty for heavy polluters such as these. This has been spun by those close to Mr Brown as a move that will burnish the Chancellor's environmental credentials.

Scepticism is called for. Mr Brown has shown scant interest in the environment before now. Indeed, green taxation has actually fallen under his tenure at the Treasury. In 1997, levies linked to pollution stood at 9.5 per cent of all taxes. Now they have slipped to 8.3 per cent. And it was Mr Brown, lest we forget, who halted the fuel escalator in the face of poujadist protests six years ago. Nevertheless, it would be churlish not to welcome the Chancellor's belated conversion to environmental protectionism, providing it is sincere, and providing the measures he puts forward are sufficiently radical to make a difference.

There will, of course, be squeals of protest from 4x4 owners at this tax increase. But they should not be taken too seriously. If people can afford a vehicle that costs at least £20,000, they can afford to pay an extra £10 a year on road tax. If anything, the new top rate should be even higher. And the macroeconomic argument for a high road taxes is perfectly sound. The principle that the polluter should pay is entirely consistent with liberal economic thinking. Road transport is one of the most substantial contributors to global warming. And 4x4s are unquestionably among the worst offenders.

But it is essential that the Chancellor avoids any hint of political tokenism on Wednesday. There must be a proper sliding scale of road tax that hits high-polluting cars and rewards low-emission vehicles. This must not be seen as simply a way to punish wealthy 4X4 owners. Ideally, the Chancellor would tell us how he intends to move towards a comprehensive road charging system, in which those who use the roads the most will also pay the most. Mr Brown should also show awareness of the disadvantages of existing incentive systems. For instance, hybrid 4x4s escape the London congestion charge, but still have twice the emissions of a small car. Manufacturers are clearly getting wise to such loopholes.

Gordon Brown and the Government will ultimately be judged by results, not intent. Despite repeated promises to bring car journeys down, traffic levels have risen by 11 per cent since 1997. If the Chancellor expects to be taken seriously in his new green garb, he will have to demonstrate on Wednesday that he understands how much lost time he has to make up for.