The system charges drivers entering or leaving the city during rush hour by scanning vehicle number plates and automatically debiting the bank account linked to each number. Stockholm officials claim to have seen a 20%-25% reduction in traffic at peak times and a reduction of more than 10% in air pollution.

The main challenge was technical . . . training a computer to get better at reading number plates, and then only Swedish-registered vehicles.

The records are discarded after three weeks, limiting the risk that government or private entities might keep track of citizens' travel patterns.

"It was quite tricky, " said Johan Westman, project spokesman for IBM Sweden, which developed the system.

Normally, optical scanning systems have only an accuracy level only around 70%. The IBM system was able to achieve 94% accuracy. For the remaining percentage where the computer isn't sure, it's referred to a human analyst to work out the plate number.

Westman says the system has made surprisingly few errors. "We've had a very low number of disputes, " he said.

Some of the changes were noticeable. Taxi revenues went up, something that may make Irish taxi unions into even more fervent supporters of congestion charging. Bus timetables had to be rewritten, because they were too fast getting to the end of the line. Best of all, Westman said, he saw the differences personally. "It reduced my own commute time by half."

Opinion polls late last week indicated that the measure was likely to pass.