Uber and Lyft have made getting places easier than ever, but their convenience appears to be having an unintended side effect on cities: more traffic.

Matter of debate: Uber says its service works alongside public transportation, helping reduce traffic. Researchers disagree. “The emerging consensus is that ride-sharing [is] increasing congestion,” Christo Wilson, a professor at Northeastern University who has studied Uber, told the AP.

Bikes, trains, automobiles? A study by the Boston-based Metropolitan Area Planning Council found 42 percent of trips taken via ride-hailing services in Boston would have been completed on public transit had the option not been available. Another 12 percent of people would have walked or biked. Plus, most people use ride-hailers end-to-end, rather than mixing the service with other modes of transport.

Why it matters: The news suggests that services like Uber aren’t complementing public transit at all, but taking people off trains and buses to put them in cars. That will clog roads and increase greenhouse-gas emissions—even if it does line the pockets of Uber and Lyft.