When roads are closed, where does the traffic go? It evaporates, say studies

The phenomenon referred to as "traffic evaporation" is gaining wider scientific resonance. Put forward by the mayor of Paris, Anne Hidalgo, it has its roots in the Braess paradox.

Seoul’s converted highway - City Clock

For more than 10 years, The Paris Region Planning and Development Agency (IAU Île-de-France) has been studying the impact of the closure of urban expressways in the centre of large cities around the world.

"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".

Paul Lecroart quotes a well-documented English study of 60 cities which calculates that when a fast lane is removed, the overall traffic decreases by an average of 14% after several months - the French magazine 'Le Parisien' reported. The reasons that lead to traffic evaporation can be many, one of which is the so-called "induced traffic": when you build a fast lane, you automatically invite increased traffic volume.   

A study commissioned by the French Ministry of Transport in 1992 estimates that after the creation of a motorway in France, the number of cars increased by 40%. This "windfall effect" disappears when the route is removed: 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 lead to ”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 organisation, moving or working remotely.

What happened when New York City decided to close 42nd Street?

Already many years ago in New York, the city's Transportation Commissioner decided to close 42nd Street, known as one of the city’s most congested streets. Contrary to expectations, not only did the traffic flow not increase but the flow improved and the traffic evaporated

The concept of traffic evaporation is directly linked to the Braess paradox. Dietrich Braess, a German mathematician, stated 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.

The paradox stated as follows:

"For each point of a road network, let there be given the number of cars starting from it, and the destination of the cars. Under these conditions one wishes to estimate the distribution of traffic flow. Whether one street is preferable to another depends not only on the quality of the road, but also on the density of the flow. If every driver takes the path that looks most favorable to him, the resultant running times need not be minimal. Furthermore, it is indicated by an example that an extension of the road network may cause a redistribution of the traffic that results in longer individual running times."

Dietrich Braess, On a Paradox of Traffic Planning, TRANSPORTATION SCIENCE Vol. 39, No. 4, November 2005, pp. 446–450  http://bit.ly/2oxttCg 

You can watch an illustration of the Braess paradox here

To read about the pedestrianisation of the right bank of the river Seine in Paris, click here

To read about the closures of New York’s 42nd Street, click here