The car-sharing lane was used during the 2024 Olympic Games in Paris and the plan is championed by the Socialist-led city hall but criticised on the right.

The peripherique ring road, known throughout Paris with a mixture of dread and familiarity as the "periph", is now reserving its leftmost traffic lane -- its fastest -- during rush hour for vehicles carrying "at least two passengers".

This excludes heavy goods vehicles but includes all public transport, taxis, emergency vehicles and people with reduced mobility.

In October, city authorities reduced the speed limit throughout the ring road to 50 km/h (30 mph) from 70 km/h (44 mph).

"We want to reduce the number of cars on the road by optimising their passenger rate in order to reduce noise and air pollution," David Belliard, deputy mayor in charge of transport, told AFP, emphasising that 80 percent of ring road users are solo drivers.

"The ring road is the busiest urban motorway in Europe and it is a real health scandal. The half a million inhabitants who live on either side of this urban motorway are exposed to levels of pollution that are still far too high, with 30 percent more asthma among children," added Dan Lert, the deputy mayor in charge of environmental issues.

The lane is signalled by white diamond signs on a blue background that are lit from Monday to Friday from 7:00 am to 10:30 am and from 4:00 pm to 8:00 pm. Police headquarters can deactivate it in case of major traffic jams.

- 'Most polluted area' -

Seven kilometres (4.3 miles) of the 35-kilometre (22-mile) "periph" in the south of Paris are currently exempt from the measure but the city is investing another seven million euros in equipment to bring the area into the scheme.

Drivers violating the new rules will be detected via artificial intelligence and will then see a message displayed on the signs asking them to change lanes.

The first fines, amounting to 135 euros, will be levied from May 1, with police responsible for analysing pictures before ordering the penalty.

With around 1.5 million daily journeys, the ring road is "the most polluted area in the capital", according to the city hall.

But there has been concern about the scheme with the right-wing head of the Ile-de-France region which includes Paris and surrounding areas, writing to Socialist mayor Anne Hidalgo to express fears over potential mass traffic jams.

"We must be able to stop this experiment, which is being implemented in such a non-consensual manner, if its results prove negative," said Valerie Pecresse, demanding an "exhaustive" assessment within three months of the impact.

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