One might think that each time a lane of automobile traffic is cut off, traffic congestion increases and that vehicles will clog other arteries. But several observations show that it is often the opposite that occurs. In many cases, automobile traffic vanishes, according to a somewhat mysterious phenomenon called “traffic evaporation”.
When, in October 2012, a fire damaged the Mathilde bridge in Rouen, France, which forced its prolonged closure, a disaster was feared. The 92,500 cars that used it every day would congest the other bridges, it was feared.
But that’s not what happened. Traffic analyzes carried out subsequently showed that 13,400 trips had vanished. “We can’t figure out where they went. As if they had evaporated,” explains Frédéric Héran, transport economist and urban planner at the University of Lille. “Obviously, people didn’t go in the clouds, but they gave up a whole series of trips. They organized their trips differently. They avoided crossing the Seine at the slightest opportunity. »
Frédéric Héran and Sabine Carette, project manager at the Observatory of Mobility and Modeling at Tours Métropole, took part, on April 26, in a webinar organized by the organization Vivre en ville to discuss the impacts of the reduction in capacity. road.
Two concepts were discussed. Traffic evaporation, or inferred traffic, refers to the reduction in road capacity that can result in a noticeable reduction in travel. Its opposite phenomenon, induced traffic, means that by adding traffic lanes to a transport infrastructure, congestion tends to increase, and motorists will take the opportunity to live further away.
“These expressions seem not very serious, but in fact, it is well documented. These are not theories, as the opponents like to say, but observations, ”explained Frédéric Héran.
The disasters apprehended
The term “traffic evaporation” originated in the early 1960s when urban planner Robert Moses, then responsible for adapting New York City to modernity, proposed increasing the road capacity of the avenue that crossed Washington Park, south of Manhattan. His project had been denounced by residents and activists, including urban planner Jane Jacobs. The park was finally closed to all automobile traffic as part of a pilot project. Robert Moses then predicted that the project would lead to massive congestion. But that was not the case. On the contrary, the whole sector has been appeased. The cars evaporated, Jane Jacobs said then.
If there is space, people will move around more. If there are none, they will move otherwise.
Frédéric Héran also cited the case of the closure of the Croix-Rousse tunnel in Lyon in 2012 for works. The newspapers had predicted a disaster, but it never happened. Which makes Mr. Héran say that the establishment of a tramway in Quebec, which will require the withdrawal of traffic lanes, will perhaps cause congestion in the short term, but in the long term, the effect will not last. “But it won’t be a disaster. Drivers will adapt. And some of them and those who can will take the tram. »
Sabine Carette for her part unveiled the results of studies carried out in the wake of the installation of a tramway in Tours in 2013. Before the arrival of the tramway, 20,000 vehicles per day used the road axis targeted by the tramway. Forecasts suggested that road traffic would decrease to 16,100 vehicles per day. But in 2017, in the end, 10,200 vehicles were counted daily on this artery. “We have halved car traffic on this axis without having made a marked reduction in road capacity”, she pointed out, pointing out that the modal share of the car had decreased by more than 4 points in favor of transport. groups between 2008 and 2019.
Caught in traffic
The evaporation of traffic is not a concept that Yves Desautels, Radio-Canada’s traffic columnist, sees frequently. Every day for a month, he has witnessed the congestion caused by the work carried out on Highway 25 south towards the Louis-Hippolyte-La Fontaine tunnel. “There is one lane less, and it’s been total hell for a month,” he summarizes. “As it’s the La Fontaine tunnel, there aren’t many other options. When you live in Boucherville, you don’t really have a choice. »
Rather, he observes a phenomenon of communicating vessels: immersed in a monster congestion, motorists fall back on other paths, such as the Jacques-Cartier bridge.
Christian Savard recognizes that the concept of traffic evaporation has its limits. He points out that highways are a “closed-loop” environment where alternatives are scarce. The work context can also create specific problems. “But when there was work on the Pierre-Laporte bridge last year, it was a lot less difficult than expected simply because people are changing their habits and taking other options,” he said.
According to him, automobile traffic could be compared to a gas. “It can contract and expand depending on the place you give it. If there is space, people will move around more. If there are none, they will move otherwise. »
It is still necessary that solutions be offered to motorists, whether new cycle paths, such as the Réseau express vélo (REV) on rue Saint-Denis, or an efficient mode of public transport, such as a tramway. To think that evaporation will take place automatically is futile, believes Jean-Philippe Meloche, professor at the Faculty of Planning at the University of Montreal.
“There is a certain mentality sometimes, in transportation planning, of saying that if we make people’s lives miserable in their travels, they will change their mobility habits. But that doesn’t always work: either they will have a more miserable life — which is not necessarily good in terms of public services — or economic activities are moved to the periphery. »
In the case of the future Quebec tramway, Jean-Philippe Meloche fears that we will not take advantage of the opportunity to transform the city. Between Sainte-Foy and Old Quebec, the tram will cross low-density areas, he points out: “It’s very beautiful and there are beautiful houses, but we can’t spend three or four billion dollars of public infrastructure and maintain the urban countryside. »
Frédéric Héran, for his part, believes that the arrival of the Québec tramway will make it possible to see the phenomenon of evaporation materialize.
“A tram is a moving sidewalk. It’s the spectacle of the city that parades in front of you. It is magic ! When you get it, a good part of your apprehensions will disappear and you will say to yourself: “Why didn’t we get it sooner”? »