a photo of traffic on the ring road outside Paris.
Les cars. (And trucks.) Charles Platiau/Reuters

The city wants to turn the Boulevard Périphérique, one of Europe’s most congested highways, into a slower, smaller, and greener “urban boulevard.”

In the ongoing campaign to reduce private car use in Paris, the next battleground is likely to be the city’s beltway. This Tuesday, Paris deputies submitted a report to Mayor Anne Hidalgo proposing a reduction of the speed limit on the Parisian Boulevard Périphérique, the 22-mile-long highway that encircles the central city, to 50 km/h (about 31 miles per hour). Cars and trucks on one of Europe’s most notoriously congested and polluted urban highways would not only be obliged to drive more slowly, they’d have less room to do it: The number of beltway lanes open to all traffic would also be slashed from eight to six. One lane will be reserved for public, emergency, and zero-emissions vehicles. The other one is to be devoted to trees.

The plan still needs to be approved by the Council of Paris in mid-June, but the mayor has already declared herself broadly favorable, and it’s likely to win. That doesn’t mean that Hidalgo can expect a smooth ride. The Périphérique is one of the key cogs in Greater Paris’ road system. Given the sustained protests against Paris’ pedestrianization of its quaysides in recent years, Paris City Hall could be in for quite a fight.

That the Périphérique needs change is no secret. Since being completed in 1973, the city’s inner ring has developed a fearsome reputation for jams. It’s a formidable smog machine, too, pumping out more than 37 percent of all nitrous oxide emissions for the Greater Paris region. And with 156,000 people living within 200 meters of the road, it discharges these pollutants in a heavily populated band of territory.

In recent years, several proposals have been floated to tame the toxic and traffic-bound Périphérique—Paris’ right-leaning opposition party proposed covering the road with a colossal concrete cap. This new report is advancing another approach: Transforming it from a limited-access high-speed highway into a slower, smaller, and greener “urban boulevard.”

The new Périphérique would be quieter too: Larger trucks whose destination was not within Paris would be banned and soundproofing at the highway’s edges improved.

Making a busy highway smaller and slower might seem like a counterintuitive means of defeating traffic in the U.S., where certain states and cities are working on widening, not culling, their traffic-clogged beltways. But during its peak hours, traffic is already moving at a very stately pace on Paris’ inner beltway: Average rush hour speeds are around 35 km/h. And many traffic experts say that lower speeds can improve fluidity and lower travel times by limiting the so-called accordion affect, in which vehicles accelerating and decelerating gradually create build-ups around junctions that turn into fully fledged jams. Driving more slowly, in some cases, can get you where you need to go faster.

Slower traffic is quieter, too—Paris noise observatory Bruitparif reckons that holding vehicles to 50 km/h should reduce average levels by a moderate but still significant two to three decibels. It is also notably safer, with slower speeds giving drivers more reaction time and lessening the force of collision impact. Past experience in Paris bears this out. In 2014, the Périphérique’s speed limit was reduced from 80 km/h to 70. This ten kilometer drop saw accidents fall by 15.5 percent in a single year.

But while there’s lots of evidence that lower speed limits can reduce congestion, accidents, and noise, their effect on pollution is less clear, mainly because there is a lack of specifically tailored research. Most gas-powered cars are more efficient at steady speeds under 50 mph, and if lower speeds reduce the stop-and-go accordion effect and let traffic flow more fluidly, the new limit could also cut the amount of pollution caused by brake pads and tire wear, a substantial source of particulate emissions. If cutting lanes creates more stop-start congestion, however, the environmental benefits might be negligible.

There’s another not inconsiderable issue: The measures proposed could prove very contentious with sections of the public. France is, after all, still reeling from the Yellow Vest movement—a nationwide protest, initially sparked by rises in fuel taxes, from mainly low-to-middle income groups who feel left behind by French government policies. Already, drivers’ associations that oppose the proposed speed limit are threatening to invoke the specter of the Yellow Vests. Mayor Hidalgo has in the past been the subject of sustained personal attacks over car-calming policies; it’s quite possible that the upcoming debate about the beltway will be even more inflamed.

At the same time, the mood in Paris (and in Europe at large) is fundamentally changing when it comes to cars and cities. Last autumn, municipalities in Greater Paris voted to ban older diesel vehicles, extending a set of policies initiated by Paris City Hall out into the more car-dependent (and less environmentally progressive) suburbs. And in last week’s European elections, support for Green parties across the continent surged. In Greater Paris, as across Europe, it is getting ever harder to damn pro-transit, anti-pollution policies as the pet concerns of a disconnected liberal elite. In gauging the reaction to Paris’ beltway transformation, it will be interesting to see if the debate, and public opinion, has indeed shifted.

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