THE DISRUPTIVE effects of ride-hailing services are apparently not limited to traditional taxi companies. In a new transportation study from the University of California at Davis, researchers confirm what many urban officials already suspected — that bus and rail- ­system passengers are also being lured away by the ease and convenience of Uber and other such services.

That's a worrying trend at a time when aging transit infrastructure is in desperate need of upgrades in a growing number of major cities. And it has prompted the nation's first proposal, by Chicago Mayor Rahm Emanuel (D), to impose a fee dedicated to mass transit on ride-hailing trips.

If approved by the City Council, the fee, which would reach 20 cents by 2019, would generate $20 million annually for the Chicago Transit Authority, which is facing enormous upcoming expenses for modernizing bus and subway services. The revenue is no game changer, but the concept makes sense and merits consideration in other cities, including Washington, one of seven metropolitan areas surveyed in the new study. (The others were Boston, Chicago, Los Angeles, New York, San Francisco and Seattle.)

The California researchers surveyed more than 2,000 people, including ride-hailing users and nonusers in those cities as well as their suburbs. They found that the new services are popular with wealthier and younger urban dwellers, and much less so with poorer and suburban ones. Moreover, their data suggested that half or more of all ride-hailing trips would not have been made at all, or would have been undertaken by foot, bicycle or transit, if not for the existence of services such as Uber or Lyft. That means the car services, rather than alleviating traffic and the pollution it produces, may in fact be adding to it — another reason to ask ride-hailing companies to contribute something to help finance transit improvements.

Ride-hailing firm officials dispute these findings, pointing to previous studies that reached different conclusions; the California survey may not be the last word. It's also true that ride-hailing services are not the only factor that may have drawn riders away from transit systems. Poor reliability and increasingly frequent breakdowns have certainly contributed to the exodus of subway passengers in the District, for instance, and low gas prices have also provided an inducement for luring people back to cars.

Nonetheless, it would be folly to ignore the growing evidence undercutting what was once the common assumption that ride-hailing apps were an unalloyed boon that would alleviate congestion. If the extreme ease and relative comfort of services such as Uber are in fact generating traffic, then they are a mixed blessing and should be treated accordingly by policymakers.

Mr. Emanuel appears to be acting on that theory, as well as on the idea that it is equitable to raise revenue from services such as Uber, which are used disproportionately by wealthier passengers, for the benefit of transit, which serves a mass clientele. That may not be the last word in the debate, but it seems informed by common sense.