The website h2020 has a very interesting examination of “traffic evaporation”-what happens to all that traffic when you close urban expressways in cities?  The Paris Region Planning and Development Agency (IAU Île-de-France) has been examining the impact of these closures in large cities around the world, and has come up with some startling conclusions.

Despite the initial fears, the removal of fast lanes does not worsen traffic conditions beyond the initial adjustments,” explains Paul Lecroart, urban planner at IAU and a specialist on this issue. “In all the cities studied, the evaporation of traffic is an important element to observe”.  Here is what is interesting-when a fast lane is removed, overall traffic decreases by 14 per cent after several months. Why? “The reasons that lead to traffic evaporation can be several, one of them is the so-called “induced traffic“: when you create a fast lane, you automatically create traffic.” 

Another study in 1992 by the Ministry of Transport in France estimated that the creation of a French motorway increased car volume by 40 per cent. Take the motorway away, and in the long term, traffic decreases. “The reduction of traffic is mainly due to behavioural change: people start adapting to the new spatial configuration. The behavioural changes that brings ‘traffic evaporation’ are: change of itinerary and of schedules, the frequency of travel, the mode of transport (shifting from cars to two-wheeled vehicles, bicycle, etc.), but also car-pooling, new family organization, moving or working remotely.”

The French term of  “traffic evaporation” is related to the Braess paradox which states” that adding extra capacity to a network may reduce overall performance and increase travel times. As in a game structure, if drivers have the possibility to choose their own route autonomously they will behave selfishly. This means that each driver will aim at improving its respective travel time by arriving first: all drivers will take the new “fast” road and will thus cause congestion.”

Take away the “fast lane” and you reduce that congestion, as seen in Paris’ work reclaiming the right bank of the Seine for pedestrians and cyclists which you can view here.  A YouTube explanation of the Braess  paradox is linked below.