• A study by Cerema (Center for Studies and Expertise on Risks, the Environment, Mobility and Planning) states that a thermal vehicle "pollutes more at 30km / h than at 50 km / h".

  • This is true, but that does not mean that lowering the general speed limit in Paris will lead to an increase in pollution.

  • In Grenoble, the reduction in traffic speed has led to a decrease in the number of cars, which has resulted in “a positive assessment” in terms of pollution, according to Benoît Hiron, a researcher at Cerema.


We saw the news spread in the headlines of the press: the speed limit to 30 kilometers per hour in Paris would be "the promise of aggravated pollution", said one. "Cars pollute more at 30 km / h than at 50 km / h in the city", recently posted a major daily, before changing its title in question. "Speed ​​limit at 30 km / h throughout Paris: pollution may increase", stated a radio. Will the new measure promoted by the mayor of Paris really increase pollution in the capital? It is more complicated than that, in the opinion of Cerema (Center for Studies and Expertise on Risks, Environment, Mobility and Planning), which produced the study that caused so much ink.

If we stick to the vehicle itself, then yes, explains Benoît Hiron, travel safety manager at Cerema.

"A thermal vehicle pollutes more at 30 km / h than at 50 km / h", in particular because the engines are optimized to run between 50 and 70 km / h.

But reducing speed has a whole bunch of impacts that also affect pollution, in particular via the number of vehicles in circulation.

"The reduction in the authorized speed allows a better sharing of public space and promotes the practice of walking and cycling rather than the use of the car", explains for example to

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Laurent Gagnepain, expert engineer in the service " transport and mobility ”from Ademe (French Environment and Energy Management Agency).

Caution

A study by Cerema carried out in Grenoble showed that the reduction in speed had reduced the number of cars in circulation by 9%, because some motorists switch to other less polluting means of transport, which are becoming more competitive in terms of rapidity when the maximum speed of the car is lowered. “We finally arrive at a positive assessment” in terms of pollution, according to Benoît Hiron, even if, indeed, driving slower does not in itself reduce pollution.

The Airparif organization, contacted by

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, is more cautious, and considers that “as it stands we cannot know if this will have an impact on air pollution”.

“The drop in maximum speed can have an impact on the number of cars in circulation (modal shift) and on the flow of traffic.

But we can only know these two consequences if we model the impact of this measure on traffic.

However the average speed of circulation is already 13 km / h, estimates Antoine Trouche, engineer in charge of the press at Airparif.

An indisputable gain in safety

But lowering the speed has many other impacts, these indisputable and unambiguous, on safety. "The issue of 30 km / h is not on the quality of the air", explains Tony Renucci, of the association Respire, whose association is nevertheless in favor of lowering the speed of circulation. “Below the optimized speed, the slower the vehicle, the more pollution it emits. (…) But in dense urban areas, in front of schools or homes, security issues are major. As well as noise pollution and the challenges of calming everyday life. And therefore, we are FOR, ”writes the association.

A pedestrian accident at 50 km / h multiplies by six the risk of mortality compared to the same accident at 30 km / h, explains Benoît Hiron.

This is why, according to this official, 30 zones are developing all over Europe (70% of the urban road in the Netherlands, 100% of single-lane streets in Spanish cities, etc.), and that a directive even has was adopted in this sense by the European Parliament in 2011. "This is the meaning of history", insists the researcher, who fears an "instrumentalisation" of data.

"When we have a shock on a person, it is neither on the right nor on the left," said Benoît Hiron, recalling that right-wing mayors, such as Boris Johnson in London or Jean-Pierre Gorges in Chartres, have put in application speed reduction.

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