The news has been reported by several newspapers: a shocking study by the MIT Senseable City Lab would reveal that the 30 km/h limit in a city like Milan would increase pollution produced by cars. However, if you look closely, this is fake news. The message that emerges from the research, carried out in collaboration with UnipolTech and presented last July 8 during the third forum The Urban Mobility Council, is in fact completely opposite: reducing speed limits has the obvious consequence of increasing average travel times in the city, and therefore also car emissions, but to a negligible extent. As the authors of the study themselves explained, the increases would probably be counterbalanced by greater use of forms of sustainable mobility and public transport (which were not quantified in the current analysis), in the face of very concrete advantages in terms of reducing accidents and deaths caused by road accidents.
I study
The research, carried out by researchers at MIT directed by Carlo Ratti, is based on data from Unipol Tech collected from over 3.4 million trips made by vehicles in the area of the Municipality of Milan over a period of 4 weeks in 2023. Analyzed, obviously anonymously, they allowed researchers to obtain important information on the movements of Milanese people, and consequently to formulate impact scenarios of a reduction in the city’s speed limit.
First of all, the data shows that Milan is already a city that almost travels at 30 km/h: the average speed of urban travel, in fact, is just 28 km/h, and that of suburban travel reaches 44. These are average speeds (which take into account stops and traffic) but they give an idea of what travel times in the city actually are. Going further, the researchers have formulated different scenarios for reducing the speed limit to 30 km/h, taking into account only the main roads, these and the secondary, tertiary and residential ones, and dividing the city into 4 concentric rings. Their calculations indicate that in the most restrictive scenario (maximum of 30 km/h on every city road) the impact would be an increase in average travel times of 89 seconds, or 7.2% more than the current scenario.
The increase in pollutants
The study also calculated the impact that reduced speeds and longer travel times would have on emissions. These are increases that, with the cars currently available, are inevitable, due to a characteristic called the “torque curve”, which identifies the speed range for which the performance of car engines is optimized. Those on the market today are calibrated to minimize emissions in the 40/60 km/h range, in line with current limits. And so by going slower, and staying on longer, they would actually emit more polluting gases. The interesting thing here, however, is how many: for CO2 we are talking about an increase of 1.5%, and for particulate matter we reach 2.7%.
Negligible percentages – explained the authors of the study – that would most likely be offset by the reduced use of private cars, in favor of public transportation and alternative mobility, which is promoted in cities at 30 km/h. For Milan, there are currently no simulations of this type, but the data collected by MIT researchers in 40 other European cities that have already lowered the limits go in this direction. And they certify the great achievements guaranteed by 30 kilometers per hour: -23% accidents, -37% deaths, -38% injuries and two and a half decibels less noise.