As well as raising hackles among drivers and conservatives, the Socialist mayor also faces an uphill battle for approval from the national government and police.

That showdown is in suspense for now, as freshly-installed Prime Minister Michel Barnier selects his ministers.

Crucial for road travel throughout the wider Ile-de-France region, Paris’s Boulevard Peripherique — known familiarly as the ‘Periph’ — is under the authority of the capital’s city hall.

READ MORE: Why the Paris périphérique is more than just a ringroad

“The 50 kph (limit) is my decision. It will happen on October 1st. We’ve been working on it for 18 years, this isn’t a new topic,” Hidalgo told broadcaster RTL.

A lower speed limit has been on the cards since January, when city hall said it would come in after the July-September Olympic and Paralympic Games that ended Sunday.

Hidalgo has also argued the reduction in the speed limit – which would decrease pollution – would also help to achieve climate goals and better public health.

Who can actually change the speed limit?

As a result of a decentralisation law passed in 2017, French law has allowed for the town hall to have power over the speed limit on its ring road, attorney Rémy Josseaume explained to BFMTV.

This means that the mayor of Paris does have the authority to pass a decree that changes the speed limit of the péripherique.

That said, there have been disagreements on this, and Josseaume noted that it would be possible for the measure to be challenged in court.

READ MORE: The key post-Olympics Paris transport changes you need to know

The transport ministry has insisted that only the government can officially change the speed limit by issuing a decree, as the city’s power does not extend to the nationwide rules of the road.

In November 2023, when the plans were initially being discussed, then-transport minister Clément Beaune promised he would not validate Hidalgo’s speed limit measure.

Meanwhile, Paris’s government-appointed police chief Laurent Nunez has also said he has a role to play. Nunez would need to organise the enforcement of the new limit, via speed cameras.

Hidalgo’s plans were decided “unilaterally” and “do not respect any of the recommendations” of a past report on the Peripherique, Conservative Republicans on the Paris city council wrote on X Monday.

Valérie Pécresse, former Les Républicains presidential candidate and the head of the Ile-de-France region called the plans to bring the speed limit down a ‘denial of democracy’, citing a survey that had found 90 percent of participants opposed to the change.

Pécresse also expressed concerns that the change would lead to an increase in traffic and pollution on other roads in the capital region.

The mayor’s Green party deputy David Belliard said in January that the lower speed limit was “in the common interest”.

A previous reduction, from 80 to 70 kph, had reduced noise pollution for residents living near the road as well as accidents, he said, citing figures from environment agency Ademe.

“Lowering the maximum speed means limiting stop-start driving (and) acceleration and deceleration effects, which makes traffic move more smoothly,” Belliard said.