by Hugh Schofield BBC News, Paris
Hundreds of police monitored traffic in Paris on Monday after high pollution levels prompted the French government to impose major restrictions.
Only motorists with odd-numbered number plates were allowed to drive.
Those with those even-numbered plates will be allowed to travel on Tuesday 18th March 2014 after the success of Monday’s initiative led to a fall in pollution.
Ministers acted after air pollution exceeded safe levels for five days running in Paris and surrounding areas.
But the environment ministry said lower traffic levels during throughout Monday and a change in weather conditions had significantly improved the smog which has descended on Paris over the past week.
The smoggy conditions have been caused by a combination of cold nights and warm days, which have prevented pollution from dispersing.
The measure has been tried once before, in 1997. Paris air quality monitoring body Airparif says it had a noticeable impact on improving air quality, although critics have disputed its findings.
Motorcycles were also covered by the ban, which ran from 05:30 (04:30 GMT) to midnight. There were exceptions for taxis, commercial electric and hybrid vehicles and for cars carrying three or more passengers.
Those flouting the restrictions faced a small fine. There was free parking for those with number-plates ending in an even number.
About 700 police ran nearly 180 control points around the Paris region, correspondents say, handing out tickets to offenders. Police were reported to have ticketed nearly 4,000 people by midday on Monday, and 27 drivers had their cars impounded for refusing to co-operate with officers.
Delivery companies are already complaining of lost income, BBC Paris correspondent Hugh Schofield says. Politically the stakes are high, with elections for Paris mayor due to start next week.
Drivers on Monday were confronted by signs warning them that those with even-numbered licence plates were not allowed on the roads
Opposition leader Jean-Francois Cope complained that the ban lacked “coherence, explanation and on the ground it’s really panic”.
On Friday, public transport was made free of charge for three days in an attempt to encourage people to leave their cars at home. This measure continued on Monday.
The capital’s air quality has been one of the worst on record, French environmental agencies say, rivalling the Chinese capital, Beijing, one of the world’s most polluted cities.
On Friday 14th March 2014, pollution levels hit 180 microgrammes of PM10 particulates per cubic metre, more than double the safe limit of 80.
PM10 particulates are emitted by vehicles, heating systems and heavy industry.
Officials say one heavy rainfall would have more effect than a one-day ban.
It is not hard to spot even-numbered registration plates on the streets of Paris. Plenty of people seem to have decided to chance it. Either they reckoned their journey was too important to cancel, or the risk of a 22-euro (£18) fine was not enough to concern them.
But overall it seems Parisians are playing the game. Most cars on the roads are indeed odd-plated, and traffic seems lighter than usual. Fewer vehicles means fewer particles, so presumably the measure is having an effect.
It is hard to criticise a measure whose aim is to protect people’s health. But there are legitimate questions over the timing of the alternate driving scheme. Pollution levels peaked at the end of last week, and were already falling. So why now?
Could the Paris mayoral elections next weekend possibly have anything to do with it? Surely not.