Because they carry multiple passengers, ride-hailed cars contribute fewer miles per person to traffic than personal cars making equivalent trips, according to a report released Thursday. That’s true even after accounting for the miles that ride-hailed drivers spend waiting for fares and going to pick up passengers, it said.

“The big takeaway is that (ride-hailing cars) are at least as efficient, if not more so, than personally owned vehicles,” said study co-author E.J. Klock-McCook, manager of the mobility transformation program at the Rocky Mountain Institute.

The energy think tank said its motivation for the study is to give cities reliable ways to assess ride-hailed cars’ impact on “congestion, convenience, mobility cost, and carbon emissions.” The impact of Lyft and Uber on traffic has become a flash point in many cities, including the companies’ hometown of San Francisco. Some 45,000 Lyft and Uber drivers work throughout the Bay Area.

The report used a year’s worth of trip data provided by Lyft for San Francisco, New York and Chicago, covering Nov. 1, 2016, to Oct. 31, 2017. Lyft did not financially back the study. Using Lyft’s data, Klock-McCook said, sets this report apart from studies that rely on rider surveys or other indirect ways of getting information.

In San Francisco, the city where Lyft has operated the longest, Lyft trips were 24 percent more efficient than those in personal cars, according to Klock-McCook. The difference was modest in the two other cities, with Lyft rides calculated as 6 percent more efficient than personal cars in Chicago and 2 percent more efficient in New York.

Here’s how the study worked: For each ride, it added up the number of miles a Lyft car spent waiting for ride orders, driving to pick up passengers, and then actually giving the ride. It assumed that each regular Lyft ride held an average of 1.67 riders, based on previous studies. For Lyft Line rides, the company’s carpool option, it assumed that each averaged 2.67 passengers when a match occurred. (Some riders who request Lyft Line rides are not matched with other passengers. In San Francisco, close to half of all rides are Lyft Line, and about half of those have matches. In New York and Chicago, about a third of rides are Lyft Line; the company would not disclose how many are matched.)

The study then compared the miles per Lyft rider to the miles per rider for the same trip in a personal car. It used the Census Bureau’s American Community Survey data to assume that the personal vehicles largely had just one occupant. In San Francisco, it assumed the personal vehicles had 1.1 occupants; in New York, 1.11; and in Chicago, 1.08.

That difference in how many people were in the cars meant that the miles per person were lower in the Lyft cars.

“The study confirms what we believe: ride-sharing can help with some of the congestion problems we see,” said Joseph Okpaku, Lyft vice president of public policy. In fact, the company was founded to address the issues of people driving alone in personal cars, he said.

The study did not consider how long the personal cars might spend seeking parking, which would have shown them being even less efficient. It also didn’t account for the fact that some of the drivers’ miles with no passengers may have been for their personal transport.

Nor did it look at the critical question of how Lyft passengers would have gotten to their destination in the absence of ride-hailing — whether they would have walked, biked, taken public transit or driven. The Rocky Mountain Institute said future studies would look at this issue, as well as the traffic impact of driver loading/unloading practices (which has sparked many complaints about double-parking in San Francisco and elsewhere).

Other researchers have drawn different conclusions about how Lyft and Uber cars affect traffic. A major study in New York last year found that ride-hailed cars averaged 11 minutes with no passengers between rides, and said this contributed to city congestion.

The San Francisco County Transportation Authority led a study last year that used special software to calculate Lyft and Uber’s impact, finding that their cars rack up more than half a million vehicle miles a day within city bounds, and account for a fifth of all vehicles miles for trips that start and end within the city.

San Francisco went to court last year to force both companies to produce details on their San Francisco rides to help the city understand their impact. Andrea Guzman, a spokeswoman for the city attorney’s office, said it is negotiating with Lyft over the release of its data and expects that to happen at a yet-to-be-determined time. The city has obtained a court order compelling Uber to provide most of its data by Feb. 14.

Both Lyft and Uber share data with their regulator, the California Public Utilities Commission, but said they wanted to shield it for privacy and competitive reasons.

“The city has broad investigatory powers, which include the power to obtain information from third parties,” San Francisco Superior Court Judge Harold Kahn wrote in the Uber court order. “That power is a prerequisite to the adequate enforcement of law.”

Carolyn Said is a San Francisco Chronicle staff writer. Email: csaid@sfchronicle.com Twitter: @csaid