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Five years have passed since Grenoble applied this 30 km/h limitation in the city and, according to a study by the Cerema specialization center (which depends on the Ministry of Ecological Transition), the number of pedestrian accidents has dropped by 24% Accidents are also less serious and the number of hospitalizations is decreasing.
On the other hand, it is not because we drive slower, at 30 km/h instead of 50, that we pollute less, on the contrary. Explanation: car or truck engines work perfectly between 50 and 80km / h. It is at this speed that they emit less CO2, nitrogen oxides and fine particles.
Below and above these speeds, engines pollute more. And, specifically, at constant speed, a car that travels at 30 km / h emits 19% more carbon dioxide than when driving at 50 km / h, calculated the center of specialization of Cerema.
This rule does not necessarily work in the city. This is where the point must be qualified. In the city, it is more complicated to make the connection between speed limit signs and air pollution. Because you 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, actually. Suddenly, the speed limit may not change anything in terms of air pollution in Paris, nor decrease or increase the number of equivalent cars.
But there is an environmental benefit to be remembered: the reduction of noise pollution. According to’Environmental and Energy Management Agency (Ademe), going from 50 to 30 km/h reduces car noise pollution by three decibels. Concretely, this means that noise is cut in half: it’s as if instead of having two speakers that transmit traffic noise in a room, we only have one.