NEW YORK – The proliferation of Lyft and Uber has snarled traffic in major U.S. cities, while ride-sharing options of Lyft Line and UberPool have siphoned off customers who would have taken mass transit, biked or walked, according new study by author Bruce Schaller, the Washington Post reports.

The study “Unsustainable?” discovered that ride-hailing services contributed to traffic jams in New York City, with Schaller warning cities of the need to counter the growth of these services before the streets are flooded with ride-hailing private cars. He found that Uber and Lyft have not decreased car ownership or personal driving, which have both seen increases, while ride share pools simply compete with subway and bus systems.

The study found that in the last six years, ride sharing added 5.7 billion vehicle miles in nine major metropolitan areas, with the upward projection “like to intensify” as the services gain in popularity. The study took place in Boston, Chicago, Los Angeles, Miami, New York, Philadelphia, San Francisco, Seattle and Washington, D.C.

“Shared rides add to traffic because most users switch from non-auto modes,” according to the report. “In addition, there is added mileage between trips as drivers wait for the next dispatch and then drive to a pickup location. Finally, even in a shared ride, some of the trip involves just one passenger (e.g., between the first and second pickup).”

When Schaller analyzed data from eight cities and California, he determined that 60% of ride-hail customers would have taken transit, biked, walked or simply not left the house. “It’s people getting out of the bus and Metro getting into sedans,” said Schaller, who has worked for the New York City Department of Transportation.

Both Lyft and Uber found fault with the report’s findings. “We strongly disagree with Schaller’s claims regarding shared rides,” said Campbell Matthews, spokeswoman for Lyft. “Just last year, over 250,000 Lyft passengers gave up their personal cars because of the availability of ride share.”