The study of nine major cities, including Seattle, says Uber and Lyft aren’t really causing people to drive less; they’re pulling passengers who otherwise would walk, take the bus or just stay home.

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The rise of Uber and Lyft in Seattle meant about 94 million additional miles were driven on Seattle-area roads in 2017, according to a new study that offers the first public estimate of the ride-hailing companies’ impact on local traffic.

People took 20 million rides in Seattle in 2017 with transportation network companies (TNCs), the biggest of which are Uber and Lyft, according to data from the Seattle Department of Transportation.

Bruce Schaller, an independent transportation consultant, took that figure and combined it with data and survey results from other cities to arrive at his Seattle estimate.

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“Without public policy intervention, big American cities are likely to be overwhelmed with more automobility, more traffic and less transit,” Schaller wrote, “and drained of the density and diversity which are indispensable to their economic and social well-being.”

Uber and Lyft, Schaller’s study of nine major cities says, aren’t really causing people to drive less; they’re pulling passengers who otherwise would walk, take the bus or just stay home.

That’s less of an issue in Seattle — where transit ridership is booming — than it is in the other cities Schaller looked at, which are mostly seeing declining transit ridership.

“A primary goal of urban mobility is to provide enough different mobility choices that people can choose to avoid the high cost of car ownership. Lyft and Uber are a big part of this equation,” said Andrew Glass Hastings, SDOT’s director of transit and mobility. “We have increasing TNC use, but also increasing transit use as well as increasing bike share and car share use.”

Uber and Lyft have grown exponentially since they first arrived in Seattle in 2014, when the City Council initially passed regulations to limit each company to no more than 150 drivers on the road at a time.

Those regulations quickly disappeared, in exactly the same way that the city’s recently passed head tax on large companies disappeared:

The City Council passed legislation to regulate large businesses. The businesses organized a signature-gathering campaign to overturn the legislation. The City Council reversed the legislation.

The city did implement licensing and insurance requirements on TNCs in exchange for dropping the driver caps.

There are now more than 28,000 licensed TNC drivers in King County, according to the division of Records and Licensing, although not all of them are active.

Uber has about 14,000 drivers in Seattle, according to a May decision from the Washington State Supreme Court. The Supreme Court is involved, because Uber and Lyft have gone to court to prevent the release of their ridership data in Seattle. The companies have also gone to court to block Seattle’s first-in-the-nation law allowing TNC drivers to unionize.

Uber and Lyft’s refusal to release Seattle-specific data makes it difficult to estimate the companies’ effect on traffic. But Mafara Hobson, a Seattle Department of Transportation (SDOT) spokeswoman said Schaller’s estimate was reasonable and noted that miles traveled in Uber and Lyft continue to increase every year.

Schaller, a former deputy commissioner of the New York City Department of Transportation, took the number of TNC rides in Seattle and combined that with datapoints from other cities:

What’s the average length of a ride? 5.2 miles.

How far does a driver travel, on average, between rides? 3 miles.

What percent of Uber and Lyft rides are shared? 20 percent.

And what would people have done had Uber and Lyft not been available? Looking at a variety of surveys, Schaller found that most TNC trips aren’t replacing car trips. Only about 40 percent of people would have driven or taken a taxi, he found. Everyone else would have taken transit, biked, walked or skipped the trip entirely.

“There’s a mixture of the good and problematic in terms of growth of Uber and Lyft,” Schaller said in an interview. “Certainly they’re offering a very valuable service, but if they took over the transportation system it would be a disaster for big cities. That process has to be managed.”

Seattle, he said, seems to be doing a better job than most cities in managing the process — with regulatory authority over TNCs and with growing investment in transit.

Uber didn’t dispute Schaller’s estimate — that TNCs have caused a 94 million-mile spurt in car driving in the city — but did push back on the idea that TNCs were a problem for cities. That 94 million miles amounts to only about two days’ worth of driving in the Seattle metro region, Uber said, pointing to federal data.

“Uber, in combination with mass transit, cycling and walking is building a reliable long-term alternative to car ownership,” Nathan Hambley, an Uber spokesman, said. Hambley said that cities should manage congestion not through caps on TNCs — which Seattle very briefly did and New York City just passed last week — but through broad-based tolling, which Seattle is also looking at.

“Congestion pricing would mean that everyone using Seattle’s roads — whether it’s a personal vehicle, delivery truck, taxi or Uber — pays their fair share to help reduce congestion and improve our public transit system,” Hambley said.

Both Uber and Lyft pointed to other factors that play a bigger role in increasing congestion — notably a booming economy, which means more people and freight are moving around and getting in each others’ way on the roads.

Lyft pointed to a New York City study that found vehicle speed began declining before the advent of Uber and Lyft.

“Bruce Schaller’s findings are directly contradicted by many academically sound studies,” Lyft spokeswoman Campbell Matthews said. “His study neglected to look at many obvious factors that contribute to congestion.”

New York City, which, unlike Seattle, has seen plummeting transit use, just passed a cap on the number of TNC drivers allowed in the city, an effort to both ease congestion and improve the financial lot of drivers.

That’s not something that’s been discussed in Seattle.