By Justin Worland
November 16, 2017

Automated cars–once a far-off dream–have in recent years left the realm of science fiction and leapt closer to the American garage. Leading U.S. automakers say that bona fide self-driving cars are coming within two decades and they’re fighting to stay competitive, from Ford’s $1 billion investment in an artificial-intelligence company earlier this year to Uber’s 2016 purchase of self-driving truck company Otto.

These advances promise relief to drivers sick of two-hour commutes and bumper-to-bumper traffic, but they leave open questions for a society shaped for the past century around the automobile. Perhaps no area is more quantifiably uncertain than the environmental impact of automated vehicles. One report from the Department of Energy found that automated vehicles could reduce fuel consumption for passenger cars by as much as 90%, or increase it by more than 200%.

That’s a significant difference given that more than a quarter of U.S. greenhouse-gas emissions come from the transportation sector, according to the Environmental Protection Agency. And scientists and policymakers say reducing that figure will be key to addressing man-made climate change.

And it’s happening quickly. Tesla, Nissan and BMW all say they will have fully driverless cars by 2021. And a report from the Boston Consulting Group suggests that by 2030 more than 5 million conventional vehicles could be replaced by automated ones.

“There’s a dramatic energy impact possible,” says Jeff Gonder, a transportation researcher at the National Renewable Energy Laboratory. “But there remains dramatic uncertainty in magnitude and even direction.”

The wide range of potential outcomes is the result of a long list of variables about how a future with automated cars will take shape. Most significantly, researchers expect that automated cars will lead to a sharp increase in the average miles traveled by a given vehicle. Key barriers to hopping in a car–fatigue, age or intoxication, to name a few–will disappear, and car owners will be free to travel further and more frequently. Workers may choose to live even further away from the office, opting to sleep in the car or use that time to squeeze in a workout. And, once in the city, car owners might instruct their vehicle to drive around in circles rather than pay for parking.

“A lot of the uncertainty comes from not knowing how the value of people’s time is going to change,” says Don MacKenzie, a researcher at the University of Washington who studies automated driving. “There will be some kind of cost associated with the travel, but it’s much less than it is today.”

Researchers have sought to model how humans might respond to automated driving using surveys, driving data and lab experiments, but ultimately the sheer number of choices and assumptions involved in transportation has made reaching concrete conclusions about driving behavior difficult.

Beyond changed driving patterns, simple technology advances will reduce the environmental toll of automated cars. Most important, engineers say that the largely accident-free vehicles can eliminate safety equipment, such as antilock brakes and airbags, that has increased the weight–and fuel consumption–of vehicles. Automated cars can also travel closer together, allowing them to take advantage of aerodynamics. Trucking fleets are already trying to take advantage of this fuel-saving measure.

Regulation represents the obvious way to protect against the potential environmental downsides of automated vehicles. The government could require cars take the most efficient route or even push consumers away from private car ownership toward ride sharing.

Write to Justin Worland at justin.worland@time.com.

This appears in the November 27, 2017 issue of TIME.

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