According to a University of Toronto paper published in June, the growth of Uber has increased transit ridership in cities across the United States, with some caveats.

The conclusions of the paper support the premise behind last-mile pilot projects on Mercer Island and in Pierce County, and backs up Lyft’s strategy to integrate ride hailing with public transit. It also validates certain parts of Mayor Jenny Durkan’s controversial proposal to reappropriate Seattle Transportation Benefit District funds towards last mile service at Rainier Valley Link stations.

The paper posits that “Uber reduces transit ridership in smaller MSAs [Metropolitan Statistical Areas] while increasing ridership in larger cities.” The paper also finds that Uber has a greater ridership effect on rail lines and in wealthier areas.

In cities with a robust transit network, Uber is more likely to become a viable last-mile option. In that context, Uber can and does amplify the marginal value of reserving exclusive right of way for transit, as “results suggest that Uber reduced commute times for public transit users while increasing congestion.”

The authors also suggest that autonomous vehicles aren’t likely to replace transit, as the Elon Musks of the world would have you believe:

While there is much speculation about how autonomous vehicles may change cities, no empirical estimates exist to date because the technology is so new. However, if autonomous vehicles make transportation more convenient, accessible and affordable relative to existing services, Uber may serve as an appropriate proxy for the estimation of such effects. Thus our results provide suggestive evidence that autonomous vehicles may complement public transit, and that this effect will likely vary across cities.

The paper isn’t completely rosy for Uber, however. It concludes that Uber can increase congestion, since it can increase the number of car trips in a metro area. The authors speculate that, since ride share drivers will sometimes cruise looking for fares, congestion similar to cruising for parking can ensue.

The study’s ambiguity puts it in the same company as other research on ride sharing’s transit impacts. While ride sharing and its car sharing cousin can reduce car ownership, they don’t necessarily reduce vehicle miles. It aligns with a U.C. Davis study that found, while long trips on transit might increase where ride sharing is present, travelers are more likely to move from transit to ridehailing for short trips, when the marginal benefit of direct, point to point travel is much greater.

Though ride sharing may present itself as the future of transportation, the reality is much less grandiose. It’s just one imperfect mode choice among many.