Public transport is free in Paris on Friday and only cars with even-numbered registration plates are allowed on the roads.
The move comes as heavy pollution levels hit the city, and follow four-straight days of free transport and traffic restrictions last week.
And it's expected to get worse on Saturday.
Police officials said they would be monitoring central Paris and 22 surrounding towns (see map below) to ensure drivers abided by the traffic restrictions.
— Préfecture de police (@prefpolice) December 15, 2016
Last week saw similar restrictions from Tuesday through to Friday as levels of fine airborne particles known as PM10 consistently above 80 microgrammes per cubic metre of air.