Adding lanes to highways often leads to a return of congestion levels, a phenomenon known as "induced traffic," according to researchers who have examined this issue in Canada and the United States.
Susan Handy, professor of environmental sciences at the University of California, Davis, explained that making driving easier by expanding roadways encourages more people to use their cars. “If driving becomes easier because there is less congestion, people are going to be more likely to hit the road,” Handy said. She noted that motorists might choose activities such as shopping or visiting others instead of avoiding travel due to heavy traffic.
While there may be short-term economic benefits from increased access and mobility following highway expansion, these effects are temporary. Some individuals might move farther from their workplaces in anticipation of an easier commute and lower housing costs. However, experts say these advantages do not last because congestion eventually returns.
Robert Noland of Rutgers University emphasized that any temporary economic benefits are negated by the environmental consequences associated with urban sprawl. “Traffic induced by highway expansion is only economically beneficial if the environmental costs of the automobile are neglected,” Noland said. He added that if drivers were required to pay fees reflecting air pollution and climate change impacts from vehicles, they would be less likely to drive just because roads are temporarily less congested. Noland also suggested alternatives: “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.”
Handy has created an "induced traffic calculator" designed to estimate how much additional congestion could result from proposed highway expansions. For instance, she cited that widening a 30-kilometer stretch near Albany could lead to 80–120 million extra vehicle kilometers traveled per year—an increase equivalent in greenhouse gas emissions to those produced annually by 6,700 cars.
Todd Litman of Victoria's Transportation Policy Institute pointed out another factor: families often move further into suburbs without fully considering transportation expenses. “Housing often costs less in remote suburbs, but we don’t take into account the cost of having two or three cars,” he said. Litman’s recent analysis looked at housing affordability in 22 U.S. cities when factoring in transportation costs; he found Houston ranks among the most expensive cities once car ownership and commuting distances are considered.
Catherine Morency from Polytechnique Montréal confirmed that Houston’s Katy Highway—a route with over ten lanes each way yet persistent congestion—is frequently cited as an example among researchers studying induced traffic effects. Morency stated this demonstrates how simply adding lanes does not resolve long-term traffic problems.
Morency is developing a tool intended for municipalities planning residential development; it aims to help households consider both transportation and housing expenses when deciding where to live. She also highlighted concerns about spatial inequities arising from expanded highways: “People [from the suburbs] who come to work in the city by car create congestion and pollution in the city,” she said. “It’s a bit like bringing our waste with us to leave it near our work.”
