Ride-hailing services such as Uber and Lyft have quickly gone from startups to billion-dollar companies and everyday parts of the urban transportation ecosystem, altering how we get around.

A new study, released today by University of California Davis transportation researcher Dr. Regina Clewlow, found that while widespread usage of these services may be decreasing the number of miles users drive themselves, it appears, overall, to increase the total miles driven in cities. App users aren’t just substituting trips, but adding additional ones. The finding raises thorny questions about future transportation policy.

Clewlow’s research found that 49 to 61 percent of ride-hailing trips “would not have been made at all, or made by walking, biking, or transit.” It’s a key factor in her determination that “ride-hailing is currently likely to contribute to growth in vehicle miles traveled (VMT) in the major cities represented in this study.”

“The implications of this data raises significant concern for travel behavior researchers, suggesting that as these services grow, vehicle miles traveled will grow, even if vehicle ownership declines,” Clewlow told Curbed. “It’s not about ownership, it’s about total vehicle miles traveled.”

The survey-based study, Disruptive Transportation: The Adoption, Utilization, and Impacts of Ride-Hailing in the United States, focused on seven major metro areas—Boston, Chicago, Los Angeles, New York, the San Francisco Bay Area, Seattle, and Washington, D.C.—and analyzed responses from more than 4,000 urban and suburban residents, who recorded their transportation and travel choices at various times between 2014 and 2016.

The study found that 21 percent of adults in these major cities used ride-hailing services, 24 percent of whom used them on a weekly or daily basis. The major reason riders gave for using these service was avoiding the hassle of parking (37 percent of respondents).

The analysis reinforced many assumptions and previous findings about the impacts of these services, also known as transportation network companies. They have had more challenges gaining traction in suburban markets, due to lack of density, and have been a huge benefit when it comes to reducing drunk driving.

But claims that ride-hailing services reduce personal vehicle ownership weren’t nearly as clear. The study found that 91 percent of ride-hailing users have not made any changes with regards to whether or not they own a vehicle, and that “those who have reduced car ownership and personal driving have substituted those trips with increased ride-hailing usage.”

A recent Reuters/Ipsos poll found that of the Americans who sold or traded vehicles in the last year, 9 percent turned to services like Lyft and Uber for their transportation needs, and another 9 percent said they planned to do the same in the next year. But again, if these riders are merely pushing personal driving onto ride-hailing services, as Clewlow’s study suggests, overall congestion and pollution aren’t being reduced, merely shifted to other drivers.

Jeffrey Tumlin, principal of Nelson\Nygaard, a transportation and planning consulting firm, says that increased VMTs show there’s a potential for significant negative economic impact on cities from these services, as people seek the comfort and convenience of a door-to-door ride via Uber and Lyft (which, as costs lower, become cost-competitive with other transit options). Streets become constrained and move fewer people.

“We have a big concern in cities that ubiquitous, cheap, door-to-door travel will result in reductions in bus ridership, walking, and biking,” he says. “The results of that is our streets will move fewer and fewer people over time. It’s arguable that we’re already seeing that in New York and San Francisco.”

Clewlow’s study also found ride-hailing led to a 6 percent reduction in transit use.

“As compared with previous studies that have suggested shared mobility services complement transit services, we find that the substitutive versus complementary nature of ride-hailing varies greatly based on the type of transit service in question,” the study concludes.

Overall, research found that ride-hailing attracts Americans away from bus service (a 6 percent reduction) and light rail services (a 3 percent reduction), while serving as a complementary mode for commuter rail services (a 3 percent net increase in use). These findings support a University of Minnesota study from last month that found ride-hailing increased utilization of subway and commuter rail while reducing the use of buses.

“The ultimate goal people want for these services is to replace personal driving,” she says.

Clewlow is optimistic that this shift can take place. Currently, she says, there isn’t quite enough coordination between public agencies and privately operated transit, but with myriad transportation agencies working with TNCs on pilot programs, she’s optimistic solutions will be uncovered.

Clewlow also says that many planners and policymakers have asked for TNCs to share their data, which would help answer bigger questions about updating transportation policy in an era of fast-moving technological changes. The data gap needs to be bridged, she says, both via private companies working with public entities, and increased research of contemporary riders and commuters and their changing habits.

“The pace of change in this space over the last five years has accelerated, and will continue to do so,” she says. “We need to work to resolve that data and technology gap.”

Curbed reached out to both Uber and Lyft for statements. A Lyft representative sent along a series of studies, including the service’s own economic impact study, a passenger survey that found that 34 percent of Lyft users would avoid owning a car because of Lyft, 56 percent use their car less, and 22 percent use Lyft to connect to public transit.

They also sent along the following statement:

"Since day one, Lyft has been focused on increasing occupancy in cars on the road. Seventy-six percent of Americans drive alone to work, and traffic congestion continues to drag down economic productivity and quality of life. We believe our cities need more efficient, affordable transportation options to make car ownership a thing of the past and look forward to continuing to work collaboratively with transit agencies to achieve this goal."

“We, along with global experts on this issue, believe that the future of urban transportation is going to be a mix of public transportation and ridesharing,” said an Uber spokesperson. “That’s why we’re partnering with public agencies around the world to deliver on this vision of the future.”

Clewlow also added that the shift from personal driving to looking at total vehicle miles traveled suggests different potential policy solutions. For instance, instead of focusing on parking and tolls as a means to regulate driving, some cities, such as New York City, have raised the idea of congestion pricing. Since users of Lyft and Uber have become more accustomed to paying per mile, she says, it may make it easier to explain and promote these types of policies, which have historically been hard to implement in the United States.