The speed limit on two-way roads without a central reservation or barrier has been lowered by 10km/hr just as French families prepare to leave for the summer holidays and foreign tourists prepare to visit.
France now has some of the tightest speed limits on this kind of road in Europe.
There are 400,000 km of them and they make up 40 percent of the French network, mainly in rural areas, sparking protests from local politicians that the limit is "Parisianist".
Motorists have staged protests - the latest being one of motorcyclists in Paris on Saturday - and launched petitions against the move but the government has refused to back down.
Despite a publicity campaign to explain the measure, an opinion poll showed 74 percent of French people opposed to it.
3,684 deaths on the road in 2017
Prime Minister Edouard Philippe has said he is ready to "accept its unpopularity", claiming that it will save up to 400 lives a year.
"The aim is not to annoy people," he said on Friday. "The aim is to ensure that there are fewer deaths and serious injuries."
Some 3,684 people were killed and 76,840 were injured in traffic accidents in 2017.
Deaths on the road went down to a historic low in 2013 but soared again from 2014-16 and 55 percent of fatal accidents were on the kind of roads affected by the lower limit, according to the road safety authority.
The government will take stock of the measure's effectiveness in July 2020.
Driving at high speed also pushes up carbon dioxide emissions, pollution and contributing to climate change.