Just keep it simple, Darling

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The only sensible way to ration discretionary spending is by price. Heating and food may be necessities but movement at the rate of one engine per person is not, least of all in cities.

Mr Darling is also right that the best form of pricing is at the margin. Make every extra journey hurt a little more, because it costs society money. It costs money in infrastructure, congestion and, ultimately, in global warming. Make the polluter pay.

But how to price? Here a different economics comes into play, that glorious, medieval summation of common sense, Ockham's Law. It states (albeit in Latin): 'Keep it simple, stupid.' Ockham's Law says that if you cannot think of a simple way of privatising the railway, leave British Rail alone. If you cannot do likewise for London's Tube, ditto. If you already have a Eurorail terminus at Waterloo, operating under capacity, don't build another at St Pancras.

Likewise, if you have a simple form of road pricing, use it. We have one, called fuel tax. It is unbureaucratic and unavoidable. A driver cannot leave home without paying it. Yet Mr Darling would no more press Mr Brown to increase petrol taxes, or the Chancellor agree, than they would fly easyJet to the moon.

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Their economists know that higher fuel taxes make every kind of sense. They cut congestion, reduce pollution and save the planet. But ministers have not got the guts to introduce them.

Political wisdom has, until recently, stood firm with Ockham. It held that Britons would rather be rationed by congestion than price. If people wanted to use a car in rush hour or at holiday weekends, the journey would take longer. Rationing by congestion may fall harder on the busy than the lazy but that is what people wanted.

Those days seem to be over. Instead, Mr Darling is claiming that he can relieve congestion, without increasing taxes. He thinks he can do so by manipulating road pricing to allow him to redirect and ration traffic nationwide and at particular times yet without driving cars the roads. He wants that planner's dream, in which every transport mode runs smoothly at just under capacity. His idea of ecstasy is a computer screen showing every motorway full of robots, gliding along at about 60mph.

Besides, he will point out, has not London shown the way? The congestion charge - which he opposed - proves that drivers can be charged for using particular areas at specific times. Why not make it nationwide?

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The answer is that London's congestion charge showed only that technology can impose a crude charge over an extensive area at a vast cost in operation and driver frustration. It can excuse some classes of drivers from paying and thus fine-tune a cut in congestion.

The chief losers are the poor, who are driven off the road, and the chief winners the rich, who have more space. Among the rich are also the consultants and operators who, in the case of London's Capita, must be the wealthiest executives in what is, in reality, the public sector.

Road pricing is so complex and intrusive - involving satellites, computers and human error - as to be unthinkable for the present. The Government cannot buy a working computer to save its life. The cost of London's congestion charge is so high that it raises hardly any revenue for public transport and 40% of fines go unpaid.

In retrospect, a better system would have increased parking fees and loading restrictions and, if more revenue is wanted, brought in supplementary licences for London driving.

That would be the best option for Mr Darling to consider. A red licence would be cheap, for residents and businesses. Others would have to be bought ad hoc, graded by price and colour.

Such a system would need no cameras or masts or computers to work, just existing traffic wardens. It could apply to all cities. If Mr Darling wanted to make such an idea national, he could relate licence discs to miles driven over a standard mileage. This would involve the vetting of mileometers, but that would be cheaper than his intrusive GPS-based system.

As for fixing different charges for different roads and different times of the day, I can only say one thing. Come back Ockham and save us. Any system designed to cut the number of vehicles on the road can only do so by making driving more expensive. The key is to make it more expensive by the easiest way possible. The key is to keep it simple, stupid.