No one knows the real cost of traffic congestion. The CBI has just put the figure at £20 billion a year — a sum suspiciously constant over the past decade. Psychologists speak of the human toll of long waits in traffic jams. Motorists demand more spending on roads but experience ever-longer delays. The only reliable forecast is that things will get worse. Now, almost three years after the start of congestion charging in London, seven other conurbations are to begin pilot projects, supported by £7 million from the Government, to see if road pricing can unclog their city centres and decongest the suburbs.

London, the pioneer, is naturally the example. The capital has shown that, to make any system work, the prerequisites are political determination, detailed traffic flow analysis and properly tested technology for vehicle identification and revenue collection. But while the charge has won broad acceptance and has been effective in deterring superfluous motorists, it has shown very clearly the shortcomings of such a crude and undifferentiated system. It makes no distinction between commuters and shoppers, and has therefore had a damaging effect on stores and businesses within the zone. It has confused the aims of reducing traffic and raising revenue, with Ken Livingstone, despite earlier denials, imposing a 60 per cent increase in July to cover the deficit on buses. And it has exacerbated congestion in the suburbs that will not be mitigated by the extension of the zone.

Despite the provision of extra buses and the welcome rise in their usage, most Londoners still see the congestion charge as a form of taxation to allow the mayor to raise money. So, unfortunately, does the mayor. As a result, the proposal for a similar scheme in Edinburgh was swiftly rejected in a local referendum. Voters saw only the penalties, not the potential gain.

Each of the seven pilot areas must be treated as a unique case. Congestion in the West Midlands — where the motorways around Birmingham are reduced to a crawl for hours every morning and evening — is very different from Cambridge, where an ancient and concentrated city centre is unable to handle the huge volume of tourists and visitors. There are also crucial variables. How good is alternative public transport? How much do local shops depend on cars? How much traffic can be diverted around the centre?

Pilot projects should look at more than a fixed charge. Towns and cities such as Brighton and Oxford already restrict cars in the centre and provide generous park and ride services on the outskirts. Where feasible, these should be extended, as long as shuttle buses are frequent and cheap. Technology should be used to vary charges according to time, season and distances travelled. Integrated transport, a national joke under Labour, must be made a reality at local level. And the private sector — taxis, minibuses and company transport — must be included. Transport takes years to plan and billions to deliver. All the more reason, therefore, for politicians to give local authorities an incentive to begin the task now.