Ride-sourcing companies like Uber and Lyft add tons of traffic to Denver and Boulder streets, and make the transportation system less efficient by cannibalizing transit, biking, and walking trips. That’s according to a study by Alejandro Henao, a PhD candidate at the University of Colorado.

Uber and Lyft famously hide their trip data from cities, so Henao went and got the data himself. He became a driver for both companies and surveyed 311 passengers over about four months in Denver, Boulder, and various suburbs. Each passenger answered 28 questions — things like where they were going, how they would’ve traveled otherwise, and demographic information.

“The main goal was to understand how people are using Uber and Lyft — what modes they were replacing,” Henao told Streetsblog. “And also, looking at it as a transportation engineer, I wanted to see how efficient this service is, compared to other services.”

About 34 percent of people surveyed said they would have either taken transit, biked, or walked instead of using the car service. That’s about 106 people who replaced transit, biking, and walking with sitting in a car, which means 106 more cars on the road that weren’t there before.

And here’s the other thing — those people aren’t adding a car to the road solely during their trip, according to Henao’s research. The mileage ticker starts when Henao is hailed by the app, continues through the pick-up, and ends with the drop-off. So if he drives two miles to pick up a passenger, then takes the passenger three miles away, the person in his back seat actually accounts for five miles worth of car traffic.

A bus going that same three miles on a fixed route carries a lot more people in a smaller amount of space, and doesn’t add excess mileage to the streets. But bad transit is also a culprit, according to Henao’s research. If the bus doesn’t come frequently or takes too long to get its destination, people will pay more for the immediacy of a personal car.

“One of the main reasons that these people stated that they’re using Uber and Lyft is because public transportation is unavailable or is poor,” Henao said.

Henao experienced the frustration of extra driving, and used his trip data to measure its effect on the overall transportation system. He examined the group of people who replaced other modes with Uber and Lyft rides, and measured the impact of those trips with Uber and Lyft and without.

Using other modes, Henao’s passengers could have moved 112 miles for every 100 miles worth of car traffic burdening the system, Henao found. With Uber and Lyft in the picture, it takes 100 miles of car traffic to move 60.8 “person miles.” In other words, more car trips moved fewer people.

Uber and Lyft “could be part of the solution for better transportation systems,” Henao says. After all, 12.3 percent of Henao’s passengers said they wouldn’t have traveled at all without the car service. They could also help cities treat their expensive and space-hogging parking addiction, Henao said.

But mostly the study is a cautionary tale to cities that think private ride-sourcing gets them off the hook for providing convenient transit. It’s not a silver bullet for mobility

“Transit agencies cancelling bus routes and subsidizing Lyft and Uber rides — you see it happening more and more all over the country,” Henao said. “But we don’t fully know the effects.”