Public transportation ridership has been declining so much in the US that some experts have called it an emergency.

The exact cause has been a point of debate: most studies have pointed to service cuts and delays as the culprit.

New research from the University of Kentucky suggests that more than anything, ride-hailing is behind the decline.

Fewer Americans are taking public transit each year.

It’s not hard to believe, either. Around the country, service is being cut and delays are rampant in old, crumbling systems like in New York City, Washington D.C. and more. All the while, fares keep climbing.

But new research could counter arguments that the decline in ridership — posited as an emergency by some transit experts — is fueled only by those service cuts. Instead, a team from the University of Kentucky has found the introduction of ride-hailing services like Uber and Lyft have had a drastic impact on cities across the United States.

"Our research finds that standard factors, such changes in service levels, gas price and auto 11 ownership, while important, are insufficient to explain the recent ridership declines," the trio of engineers writes in the paper, presented last week at the Transportation Research Board's annual meeting.

"Our results also suggest that for each year after Transportation Network Companies (TNCs) enter a market, heavy rail ridership can be expected to decrease by 1.3% and bus ridership can be expected to decrease by 1.7%."

Using a random-effects model (similar to a common regression analysis) the team set out to look at ride-hail and transit trends in 22 of the US' largest cities. The study accounted for 10 different variables, including population, density, proportional car ownership, gas prices, and transit miles in each metro area.

Professor Gregory Erhardt, an assistant professor of civil engineering and an author of the study, says the question was catalyzed by a City Lab article from last year. That essay stated something quite bluntly of buses: "More Routes = More Riders," citing research from Canada's McGill University which said service cuts were behind declining bus service.

Ride-hailing effects are so substantial that, if the current trend continues, bus service in cities studied could be hit nearly 13% over the 8 years. And Erhardt says it won't be a problem easily fixed by just adding more buses on streets.

"We found that wasn't the case, there's something else going on" Erhardt said in an interview. "In order to offset the magnitude of this, you would need to increase bus service by about 20%, and a lot of transit agencies simply aren't going to have the resources to do that."

And even if its a faster, and sometimes possibly even cheaper, option than taking a bus — ride-hailing simply can't replace public transit due to space constraints.

"When you take those people out of a bus and into an Uber," said Erhardt, "You increase traffic and congestion for everyone, and even slow that original bus down."

Some cities, including Los Angeles and New York, have considered a congestion tax on drivers for entering downtown areas to help combat this issue. Other studies, like one by San Francisco's county transportation authority, have found that ride-hailing services also increase traffic congestion.

To that end, Uber has supported congestion pricing in cities that are considering it.

"While studies disagree on why US transit ridership is declining, everyone agrees on the solution," it said in a statement. "We need tools that help ensure sustainable travel modes like public transportation are prioritized over single occupant vehicles. That’s one reason we believe in comprehensive congestion pricing, which would provide millions to invest in cities’ public transportation systems."

Its closest competitor, Lyft, has partnered with many transit systems and even included public options inside its app in many cities. A spokesperson said that it’s committed to transportation equity for all riders.

"Lyft was founded with the mission to improve people’s lives with the world’s best transportation and many of our most popular pick-up/drop-off spots are adjacent to public transit stations, indicating that Lyft is complementary to public transit," it said in a statement. "Lyft is committed to effecting positive change for our cities by promoting transportation equity through shared rides, bikeshare systems, electric scooters, and public transit partnerships."

The final solution, though, won't be in one policy effort. Erhardt hopes his team’s research might provide useful tools as policymakers debate solutions to public transit that hemorrhaging riders, and streets that will only become more congested with time.

"The exact scope of those policies is a point of discussion," he said. "It might include congestion pricing or changing how we allocate right-of-way between cars, buses, transit, bikes, and pedestrians.

"These are the sorts of discussions we ought to be having at a policy level - its not as simple as adding more buses."

Do you work at Uber, Lyft or another ride-hailing company? Got a tip? Get in touch with this reporter at grapier@businessinsider.com. Other secure contact methods are available here.