Windscreen sensors will pay the C-charge

Motorists driving into London's congestion zone are to get windscreen sensors that automatically pay the charge.

The hi-tech system could consign to the history books the inconvenience of paying by phone, website or vending machine each time you enter the zone.

Officials working for the Mayor are to test the technology in secret trials beginning next month.

A hundred drivers will take part in the experiment in south London-The technology works in three stages:

If trials are a success the "tag and beacon" system could be launched in the congestion charging zone - and the proposed western extension - in 2009.

As well as being more convenient, TfL says the tag and beacon scheme might also be cheaper. Instead of paying a fixed charge to enter the zone the new system will charge motorists on a sliding scale - depending on the type of vehicle, time of day, location, and time spent in the zone.

Motorists driving out of the city during congestion charging hours could travel free, while those heading in could be charged.

"It would be a far simpler, and fairer, means of levying the charge," said congestion charging director Michele Dix.

Successful small-scale trials of tag-and-beacon have already been carried out on Old Kent Road and

Commercial Street. Next month, at an undisclosed location straddling part of the existing congestion zone, trials of the system will begin in earnest, lasting until the autumn when officials will compile a report.

It is thought that the tags will cost between ?5 and ?50. Drivers may have to pay to have them fitted. In return they may receive inducements such as "free" days in the zone.

Tag-and-beacon will run in parallel with the existing system for several years, as motorists visiting London only occasionally will not want to have tags. But TfL believes the incentive of more finely-tuned charges will encourage others to have them fitted.

Enforcement cameras will remain to catch motorists trying to cheat the system. The cost of the new system is not yet known, but is certain to run into several million pounds.

And as drivers sign up, TfL will rake in less money in fines. However, proposals to increase the charge and extend the zone should offset the expense.

National road-pricing based on London's tag-and-beacon scheme could be launched in four to seven years, instead of the 15 it would take for a satellite-based system, he said.