Is it true that when you widen a highway, congestion returns to the same level after a few years?
Maude Martin
Yes, adding roadways attracts more cars and perpetuates congestion, according to four researchers who have studied the issue in Canada and the United States. This is called “induced traffic”.
When you add traffic lanes, “if driving becomes easier because there is less congestion, people are going to be more likely to hit the road”, says Susan Handy, a civil engineer and professor of environmental sciences at the University of California, Davis (UCD). For example, motorists will go shopping, go to a restaurant or the cinema, go visit someone, rather than avoiding hitting the road because of congestion. ”
In the short term, there may therefore be a positive effect on the economy.
Some might decide to move further from their workplace, where properties are cheaper, if they expect to be able to easily drive back and forth.
But these potentially beneficial effects on access to housing and economic activity do not last, since congestion eventually returns.
Additionally, temporary benefits are entirely offset by the environmental impact of urban sprawl, according to Robert Noland of Rutgers University in New Jersey, who has studied the issue for 30 years.
Traffic induced by highway expansion is only economically beneficial if the environmental costs of the automobile are neglected.
Robert Noland, Rutgers University expert
If motorists had to pay fees to offset the costs of air pollution and climate change caused by motor vehicles, they would not take their cars to take advantage of less congestion, Mr. Noland. Additionally, “if there is space for a railway along the highway, for example, it is cheaper to transport people by public transit than to widen a highway”, he says.
Susan Handy has developed an “induced traffic calculator” to predict the increase in congestion generated by the planned highway expansion.
The calculator gives its results based on global warming. For example, widening a highway 30 km near Albany would generate an additional 80 to 120 million vehicle kilometers per year, which equates to 0.2 to 0.8 million tonnes of CO2, or the annual greenhouse gas emissions of 6,700 cars.
Consult the UCD induced traffic calculator (in English)
another element that contributes to induced traffic: households are more inclined to settle in the suburbs because they underestimate their transportation costs, recalls Todd Litman, who heads the Transportation Policy Institute in Victoria, Colombia British. “Housing often costs less in remote suburbs, but we don’t take into account the cost of having two or three cars”, he notes.
A 20-lane highway in Houston
MR. Litman recently published an analysis of housing costs in 22 U.S. cities, taking into account transportation costs. “Houston is often cited as a city with a large population growth without housing prices being that high, but when you consider the costs of owning cars and traveling great distances every day, this is no longer the case”, says the researcher. Houston ranks fourth among the most expensive cities in the United States.
Catherine Morency, transport specialist at Polytechnique Montréal, confirms that the example of Houston is often cited by researchers working on “induced traffic”. We often talk about Houston’s Katy Highway, which has more than ten lanes in each direction but is still congested. It’s a demonstration through the absurd”, she points out.
The Montreal researcher is working on a tool intended to help municipalities choose where to develop residential neighborhoods, which would also allow households to choose where to settle, taking into account transportation and housing costs. This tool will have several components for different clienteles.
Ms Morency adds that widening highways, by promoting urban sprawl, creates “spatial inequities”. “People [from the suburbs] who come to work in the city by car create congestion and pollution in the city”, which the urban population must endure. “It’s a bit like bringing our waste with us to leave it near our work”, she illustrates.