Smart roads: why speed limits are not enough to stop traffic
As recent research shows, urban design is more important for regulating city life than traffic restrictions
In an era marked by increasing political polarisation, even urban planning is falling into the crosshairs of the so-called culture wars.
It all started a few years ago with the concept of the 15-minute city, the idea of neighbourhoods in which essential services can be reached on foot or by bicycle in a quarter of an hour and soon became the focus of numerous conspiracy theories. More recently, the 30 km/h urban speed limit has fuelled new cultural clashes. However, the results of our research at the Senseable City Lab, in collaboration with UnipolTech, suggest that these battles may be misguided. In fact, the best way to reduce car speeds in the city is not so much the number indicated on road signs as the design of the road.
To better understand this phenomenon, let us take a step back. In 2020, the proposal to limit speed to 30 km/h in cities became a central theme in the re-election campaign of the mayor of Paris, Anne Hidalgo. The idea was simple: slower traffic means safer streets, quieter neighbourhoods and cleaner air. These beliefs were also supported by our previous research, conducted in Paris itself, which showed that slower urban areas attract almost twice as many people, promoting greater diversity and social mix.
As soon as she was re-elected, Hidalgo introduced the new speed limit, and soon other cities - from Milan to Brussels, from Bologna to Amsterdam - began to follow suit. The backlash was not long in coming, especially in Italy. For many drivers, such a low limit seemed more a form of social control than a road safety measure. In an era dominated by online clicks, the measure was immediately interpreted as a sign of authoritarian drift.
Our Lab also ended up in the crosshairs. In July 2024, we presented the results of a study on the impact of 'Zone 30' in Milan. Using telemetry data provided by UnipolTech, we accurately assessed the effect of such a policy. Result? The increase in travel time would have been negligible, around 34 seconds more per journey. The reduction in road accidents would have been around 37 per cent, as quantified by previous studies. While pollution levels would have remained essentially unchanged, with a minimal and statistically insignificant increase.
