The results surpassed the authorities' initial goal of reducing traffic by up to 15 percent, the study from Stockholm's city government showed. It also has helped increase the number of people using public transport by about 4.5 percent.

Car pollution in downtown Stockholm was cut by between 9-14 percent, the study said.

The trial ends in July but the comprehensive analysis was released Wednesday, as traffic always slows down during the Swedish summer, organizers said.

The tolls were hotly contested when the trial started in January, with opinion polls showing most people thought it was a bad idea.

But public opinion later swung in favour of the congestion charges, and a May survey had 62 percent of Stockholm residents saying they would vote "yes" to making the system permanent in a September referendum.

Depending on the time of day, Stockholm drivers have to pay between 10 kronor and 20 kronor (about 1 to 2 euros) when they enter or exit the city's centre. The toll is in effect from 0430 GMT to 1630 GMT every weekday, with no fees on weekends, holidays or at night.