Paris ring road at 50 km/h: pros and cons, we take stock

The lowering of the speed limit from 70 to 50 km/h on the Paris ring road, planned by the Paris City Hall for entry into force on October 1, 2024, is strongly criticized. It must be said that no impact study has been carried out and that one can therefore doubt the benefits announced. Here is what one can expect from reading various studies on the subject.

Towards longer journey times?

The first question to ask, because it is decisive for the other benefits announced, is the impact on average speed. Indeed, lowering the speed limit on a congested road can ultimately increase the speed of travel of users by limiting the accordion effects. This is already what happened when the limit was reduced from 80 to 70 km/h on the Paris ring road in 2014. Thus, the Paris City Hall and the Police Prefecture estimate that this lowering of the limit has allowed “an improvement in average traffic speeds”. “Motorists also benefit from a travel time saving of around 15% in the morning and 5% in the evening”according to the Municipality and the Police Prefecture.

However, there is no evidence that lowering the speed limit further will have the same effect. While the average speed on the Paris ring road is already below 50 km/h, this is far from being the case at all times of the day and night. In a 2014 study by Ademe (the French agency responsible for ecological transition), entitled Impacts of speed limits on air quality, climate, energy and noisethe authors questioned the positive impact of lowering the speed limit below 70 km/h. While the agency indicates that a “changing the speed from 80 to 70 km/h on a congested road generally improves traffic flow”and therefore reduces air pollution, she also notes that“below 70 km/h, this effect is rather negative. In practice, the situation is more complex since it is necessary to take into account the effect of the speed limit on congestion” Furthermore, some motorists might be tempted to use internal roads, further congesting the capital.

A drop in pollution is likely, but not phenomenal

Driving at 50 km/h rather than 70 km/h may have a slight impact on fuel consumption, and therefore on pollutant emissions, but it is not achieved automatically by lowering speed limits. Here again, everything will depend on the impact on the actual speed of motorists and, above all, on congestion on the ring road. Ademe concluded that “The analysis of the real impacts on air quality of speed limits tends to show gains for speed reductions at high speeds, and a much more contrasting situation for speed reductions at low speeds, in particular the reduction from 50 to 30 km/h”.

Similarly, while fine particle emissions from our car tires and brakes can be reduced by lowering speeds, it is mainly the accordion effects, which promote braking and acceleration, which have an impact on the wear of these parts.

For Karine Léger, director of AirParif, the organization that measures air quality in Île-de-France, “The reduction alone of a limit to 50 km/h instead of 70 km will have a very limited impact on air pollution”she said to West France. No reason to hope for a significant reduction in air pollution, then.

Reduction of noise pollution: the main objective of this measure

Although the drop in air pollution may not be very significant, the Paris City Hall teams hope to provide more relief to local residents' ears. “Noise pollution is poisoning the lives of residents near the ring road”explains David Belliard, deputy mayor of Paris in charge of the transformation of public spaces, West FranceIn 2014, the reduction of the maximum speed of the ring road from 80 to 70 km/h effectively allowed a reduction in noise pollution of 1.2 dB at night and 0.5 dB during the day, a figure Noiseparifthe noise observatory in Île-de-France.

However, as Ademe points out, “Speed ​​limits do not always lead to a reduction in noise levels. For low traffic speeds, certain elements may then appear to generate road noise: development, road surfaces, high flow rates and nature of traffic…” While some sections of the 35 km of the Paris ring road have recently benefited from resurfacing, others could do with a new surface.

A logical but not automatic reduction in the risk of accidents

Finally, lowering the speed limit on the Paris ring road could limit the risk of accidents. Naturally, these are more frequent at a higher speed, and above all much more dangerous. Between 70 km/h and 50 km/h, the impact can be half as violent, indicates Cerema (Centre for Studies and Expertise on Risks, the Environment, Mobility and Development). However, a 20 km/h reduction in the speed limit does not lead to a reduction of the same order in the effective speed of users.

Moreover, according to figures from the Police Prefecture that Caradisiac was able to obtain, the accident rate has changed little since the speed limit was reduced from 80 to 70 km/h. The number of accidents was certainly down by almost 18% over the period from 2014 to 2018 compared to that from 2008 to 2013, but the number of hospitalized injured people fell by less than 2% and the number of deaths even increased by a little over 6%.

In short, the announced benefits of this measure are difficult to quantify and could be quite moderate. We will certainly have to wait for an initial assessment to see more clearly.