Uber and Lyft may be revolutionizing the ride-hailing industry, but they’re not doing much to ease traffic congestion. In fact, a new study shows the two companies have the opposite effect: They’re adding to congestion.

A new report from the University of California, called “Disruptive Transportation: The Adoption, Utilization, and Impacts of Ride-Hailing in the United States,” showed ride-hailing services may be reducing the use of public transportation, adding more cars to the road.

“We found that a large portion of travelers are substituting ride-hailing in place of public transit, biking, and walking trips,” study co-author Regina Clewlow wrote in Planetizen. “These trips, which are being substituted for or generated by ride-hailing, are most likely adding vehicles to the road in major metropolitan areas.”

The study is the first of its kind, providing data from major American cities. It shows a 6 percent reduction in transit use in major American cities.

“It doesn’t matter who owns the vehicle,” Clewlow told Wired. “It matters how many miles are driven on the road. So me swapping out a trip that I would have driven myself for a trip that I’m riding in is still the same number of miles.”