The northern Italian city of Milan is banning all traffic from its streets for 10 hours on Sunday (Monday NZ time) in an attempt to reduce smog.
The measure, first imposed on a trial basis in 2007, is triggered whenever pollution exceeds the statutory limit of 50 micrograms of particulates per cubic metre of air for 12 consecutive days. The full ban was last enforced in February.
Satellite imagery shows Milan to be one of the most polluted cities in Europe, the BBC reports.
An estimated 120,000 vehicles will be affected by the move, according to the Corriere della Sera newspaper.
The most polluting vehicles have been banned from driving through the city centre since Thursday, but on Sunday no traffic at all will be permitted between 8am and 6pm local time
The move is not popular with all environmentalists, who argue that the city's public transport system should be improved to discourage people from using their cars.
Local Green Party councillor Enrico Fedrighini says cars with three or four people inside should be offered free parking, for example.
"One or two car-free Sundays each month won't do anything to tackle the smog crisis," he told Corriere della Sera.
Public transport will be bolstered during the day, with extra metro trains and buses operating.