Traffic jams at 8.00am were cut by half to 87 kilometres, although by 9.00am they had reached 123 kilometres, according to road safety officials.
But the number of recorded traffic offences went up - to 4,500 by 10.30am, 3,000 of them for breaking the ban on cars with even numbers of their licence plates.
Only cars whose registration ends in an odd number were allowed on the roads of Paris and 22 towns around it on Monday to try and reduce historically high fine-particle pollution that has plagued the French capital for over a week.
Motorists who breached the ban were liable to fines of 22 euros and could have their vehicle seized with 700 police officers manning about 60 checkpoints.
Ecology Minister Philippe Martin was to decide at midday whether to continue the measure on Tuesday, when the restriction would apply to cars with even numbers.
Electric and hybrid vehicles, cars with more than two passengers, emergency service vehicles, foreign-registered cars, driving school cars and taxis were exempt, as were drivers who were journalists with press cards, sales representatives and vans used for work.
The measure came on top of an already existing ban on heavy good vehicles, except for refrigerated lorries, and a tighter than usual speed limit.
Public transport in the Paris region has been free since Friday.
A cloudy sky raised hopes of a fall in smog caused by over a week unseasonably sunny days with little wind to disperse exhaust and heating system emissions.
Alternating registration numbers has only been tried once before in France, in 1997 when it reduced fine-particle pollution by 15 per cent, according to an international inquiry
published in 2013.
It has been applied in other countries, including in Athens and Rome, with mitigated success.
"Experience abroad has been very varied," lung specialist Christos Couaid, who is vice-president of France's committee on respiratory diseases, told RFI. "I think it lowers pollution temporarily. It has to be part of a wider plan of taking account of air quality."
Housing Minister Cécile Duflot, a member of the Green party EELV, criticised the government for not having gone "further, quicker" to fulfil President françois Hollande's campaign promise that France could become a "nation of environmental excellence" on Monday morning.
"Perhaps we need a speedier reaction and we doubtlessly need to inform the population better," she told the BFMTV television channel.
Far-right leader Marine Le Pen slammed the government for "tracking down" motorists while France becomes "a country of urban riots".