Uber and Lyft have long argued that ride-hailing apps have the potential to make cities better by ameliorating traffic and reducing personal car ownership. But there is a growing body of research that suggests the opposite is taking place. The latest study, published Wednesday in the journal Science Advances, underscores how Uber and Lyft are worsening traffic in the city that gave birth to the ride-sharing phenomenon.

San Francisco, home base to both Uber and Lyft, is experiencing some of its worst traffic in years — research firm INRIX ranks it the eighth most congested city in the US — and much of it is due to the rising popularity of transportation network companies (TNC, an industry term used to describe ride-hailing apps like Uber and Lyft). How bad is it? According to today’s study, between 2010 and 2016 traffic congestion in San Francisco increased by about 60 percent — and Uber and Lyft are responsible for more than half of that increase.

Uber and Lyft are responsible for more than half of the increase in vehicle delays

The study began when the San Francisco County Transportation Authority (SFCTA) reached out to Gregory Erhardt, associate professor for civil engineering at the University of Kentucky and an expert in transportation models and travel forecasting, to help determine the impact of ride-sharing on the city’s traffic patterns. (The authority previewed the findings of the study in its own report released in October last year.)

Erhardt said the goal was to present a “before” and “after” picture of San Francisco’s streets — before Uber and Lyft became popular in 2010, and after they became a dominate mode of transportation in 2016. To start out, they used a standard transportation simulation to control for variables, such as population growth, changes to the city’s transportation system, and a rise in freight and deliveries.

A big challenge going into this project was access to data. Uber and Lyft are sitting on troves of really interesting data on the movement of cars and people, but they are generally reluctant to share it with governments or academic researchers out of privacy concerns and for fear of compromising their competitive advantage. As a result, the research into the effect on congestion has been a mixed bag: some studies conclude that TNCs reduce congestion, while others note they increase vehicle miles traveled (VMT). Still others fail to reach any conclusion at all — and most researchers cite the lack of data as a prime obstacle.

A big challenge going into this project was access to data

Erhardt and Joe Castiglione, the deputy director for technology, data, and analysis at SFCTA, reached out to some data scientists at Northeastern University who had put together a computer program that queries the API (application programing interface) for both Uber and Lyft, and reports the location of the 10 closest vehicles in both apps.

“And what they were able to do is set up sort of a grid of client calls to the API across San Francisco for a six month period in late 2016,” Erhardt told The Verge. “Every two seconds, they say, ‘Hey, where are the 10 closest vehicles, write that to a database. And we ended up with about 17 terabytes worth of data. But what you get out of it are these traces of where the vehicles are, where the drivers are, and when they’re available for a ride.”

Erhardt’s team used a panel regression model to determine the change in travel time during this period, and what they found surprised them: far from alleviating traffic, Uber and Lyft are actually the “biggest contributor[s]” to San Francisco’s worsening traffic congestion.

Specifically, they found that the difference in travel times in congested conditions versus travel times in a free-flowing scenario — which they characterize as “vehicle hours of delay” — increased by 62 percent. Average speeds in San Francisco decreased by 13 percent in the time period. By contrast, in a simulated model that removes Uber and Lyft from the equation, weekday vehicle hours of delay increased by only 22 percent and average speeds decreased by 4 percent in the city.

Average speeds in San Francisco decreased by 13 percent in the time period

“There has been this narrative about how [Uber and Lyft] have the potential to support transit, bringing people to and from transit stations, they have a potential to enable carpooling, and they have some potential to reduce people’s auto ownership,” Erhardt told The Verge. “And those things are true to a degree. But the question is whether they’re true enough to offset the ways in which TNCs increased congestion. And we find that when you look at the data, that’s not the case.”

Erhardt’s team isn’t the first to look at the effect Uber and Lyft have had on traffic in big cities. Probably the most widely cited study was done by Bruce Schaller, a transit consultant who served as deputy commissioner for traffic and planning in New York City. Last summer, he released a report called “The New Automobility,” in which he concluded that ride-hail companies like Uber and Lyft have added 5.7 billion miles of driving annually in cities like Boston, Chicago, Los Angeles, Miami, New York, Philadelphia, San Francisco, Seattle, and Washington, DC.

Erhardt said his research differs from Schaller’s in so far as it “zoomed in” on a specific city during a specific period time, and examined the effect of Uber and Lyft on traffic congestion. “To be able to look at this in two different ways, and find similar results, is really important and really encouraging,” he added.

Last week, the Sierra Club launched an ad campaign urging Uber and Lyft to curb air pollution by switching to fully electric cars. The environmental group is hoping to leverage Uber’s impending IPO to highlight the ride-hail companies’ contribution to climate change and rising carbon emissions.

To be sure, Uber and Lyft have weathered criticism about pollution and traffic congestion for years. And the company has tried to address it through a variety of means, including its bike- and scooter-sharing services, its effort to integrate public transportation scheduling and ticketing into its app, and its incentive program to get drivers to switch to electric cars. Uber also supported New York City’s recent push for congestion pricing.

spokespersons for both Uber and Lyft sent similar-sounding statements

Asked for a comment on the study, spokespersons for both Uber and Lyft sent similar-sounding statements touting their support for “comprehensive congestion pricing,” improved bike and scooter infrastructure, carpooling, and support for public transportation. They also both tried to cast doubt on the study’s findings. Uber said that “studies disagree on causes for congestion,” while Lyft notes that Erhardt’s study “overlooks notable contributors to congestion including increased freight and commercial deliveries, and tourism growth.”

Erhardt said he hopes his study will serve as a wake-up call to city planners, who have the power to enact change that can help reduce the congestion. “If I’m riding in an Uber or Lyft, it’s pretty convenient, it’s reasonably cheap, I get door to door service,” he said. “However, there’s an externality, there’s an effect on other people on the road. Me being in a car, I’m causing congestion for other people. And the same thing happens if I’m driving my personal car as well. And so what city planners and city officials need to do is somehow balance that out, balance out the benefits of the person in the car, with the cost to everyone else.”