Closing the Seine right bank
A bitter controversy arose after Mayor Anne Hidalgo planned to close the road parallel to the right bank of the Seine river in order to improve air quality and reduce traffic. Oppositions include the motorist groups and the right wing. The motorist groups accused the socialist administration of a “vendetta against drivers”. The right wing claimed that there wasn’t enough evidence that the closing of the road would improve air quality. Traffic might just be diverted to the surrounding areas and increase air pollution and traffic there instead. Mayor Hidalgo responded with the following remark, “Of course, we don’t have a crystal ball, but we are convinced that if there are traffic jams in the short term, they will evaporate in the long term as people change their habits and use other forms of transport.”
So what does Mayor Hidalgo’s statement actually mean? We can use network traffics and Braess’s paradox to understand her statement. The Braess paradox states that sometimes adding new paths in a network can increase travel time. In other words, removing paths can sometimes reduce travel time, but how does this actually work? Consider the transport system in Paris as a traffic network with two types of edges, those with weights depending on the number of commuters and those with weights that do not depend on the number of commuters. The first type of edges includes roads for cars, and the second type of edges includes the public transit system and sidewalks. When the Seine’s right bank gets closed, initially the traffic would increase. However, once more people start using other forms of transport aka switch from the first type to the second type of edges, the total travel time of the system might decrease because the second type of edges can take on more commuters without increasing travel time.