Since this morning: the speed is limited to 30km / h in Paris, with the exception of certain major roads. Almost 200 French cities already apply this speed limit. A limitation which lowers the number of accidents but not air pollution.

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Grenoble has been applying this 30 km / h limitation in the city for five years now and according to a study by the Cerema center of expertise (which depends on the Ministry of Ecological Transition), the number of accidents involving pedestrians fell by 24%. Accidents are also less serious, and the number of hospitalizations is declining.

On the other hand, it is not because we drive less quickly, at 30 km / h instead of 50, that we pollute less, on the contrary. Explanation: Car or truck engines work optimally between 50 and 80km / h. This is the speed that they emit the least CO2, nitrogen oxide and fine particles. Below and above these speeds, engines pollute more. And concretely, at constant speed, a car traveling at 30 km / h emits 19% more carbon dioxide than when traveling at 50 km / h, calculated the center of ?? ? Cerema expertise.

This rule does not necessarily work in the city. That’s where we have to qualify the point. In the city, it is more complicated to make the connection between speed limit signs and air pollution. Because we never drive constantly at 50 km / h, nor at 30 km / h. We brake and accelerate a lot, which means that the average speed in Paris is already 15 km / h, in fact. Suddenly, the speed limit may not change anything in terms of air pollution in Paris, neither decrease nor increase in the number of equivalent cars.

But there is an environmental benefit to remember: the reduction in noise pollution. According to the Environment and Energy Management Agency (Ademe), going from 50 to 30 km / h can reduce noise pollution from three-decibel cars. Concretely, this means that the noise is divided by two: it is as if instead of having two loudspeakers which broadcast traffic noise in a room, we donâ € ™ t ???? had more than one.