Elon Musk announced last week that the Model S will feature "autopilot," an ability to take over for the driver in limited situations. The move closes a gap between Tesla and more established automakers, which have provided increasing levels of autonomous tech. It will net the young company quite a windfall and make driving safer and easier for its customers. It’s a significant step toward a future where cars drive themselves, and doing so with electricity offers an enticing view of a future where cars are awesome and the atmosphere is squeaky clean.

Unless the new Tesla and other autonomous vehicles end up doing more harm than good to the planet.

As driving becomes less onerous and computer-controlled systems reduce traffic, some experts worry that will eliminate a powerful incentive—commuting sucks—for living near cities, where urban density makes for more efficient sharing of resources. In other words, autonomous vehicles could lead to urban sprawl.

It's simple, says Ken Laberteaux, a senior scientist at Toyota. If you make transportation faster, easier and perhaps cheaper, then people won't mind commuting. "What a consumer is expected to do is see what they can gain by moving a little further from the job centers or the cultural centers,” he says. That’s bad news: Urban sprawl is linked to economic, environmental, and health hardships.

Proponents of autonomous cars predict that once humans are no longer responsible for driving, we’ll see a decrease in crashes, a reduction in congestion and a spike in ride sharing. The current model, in which people own or lease a car, could fundamentally change to a subscription model in which people simply summon a car when they need one, and it appears, all by itself. Laberteaux says such a shift is possible, but won’t happen for a long time.

Self-driving cars might one day lead to a decrease in personal car ownership, and an increase in walking-oriented lifestyles. But first, we’ve got to live through the phase of semi-autonomous driving. Those cars will likely need human intervention at least some of the time, so they won’t change the current ownership model. They will, however, make driving way more convenient, incentivizing long commutes.

Elon Musk Ariel Zambelich / WIRED

Laberteaux’s not the only one concerned about this. Autonomous vehicles should ease highway congestion, and commuters will be able to catch up on work or sleep en route to the office. That limits the incentive to trade your McMansion for a brownstone, says Reid Ewing, director of the University of Utah’s Metropolitan Research Center. The implications of this go beyond transportation; in a 2014 report for advocacy group Smart Growth America, Ewing linked sprawl to obesity and economic immobility.

Ewing likens autonomous driving to the construction of “superhighways” during the post-war boom years, which spurred suburbanization. “If you can travel at higher speeds with less congestion and you can use your time productively while you’re traveling in a self-driving car, the generalized cost of travel will be less on a vehicle-per-mile basis,” says Ewing. “Just like when, before the interstate system, people were traveling at 30 miles per hour, there wasn’t nearly the spread of development that there is today.”

Tesla owner Al Pace driving through Palo Alto. Ariel Zambelich / WIRED

What’s interesting is that the potential for increased suburbanization comes as many American metropolitan area are seeing a reduction in sprawl, in large part through a city-loving “Echo Boomer” generation coming into the workforce. According to a report Rutgers University published last month, population growth in New York City since 2010 has surpassed its suburban communities for the first time since World War II.

So how can urban planners ensure that trend toward urbanization doesn’t reverse again? Use traditional tools like zoning, pricing, and urban design, says Ratna Amin, director of transportation policy at the Bay Area planning nonprofit SPUR. “Transit sprawl and autonomous vehicle sprawl, these things happen in the absence of growth management,” says Amin. “We can put boundaries on sprawl.”

SPUR, like like-minded organizations, advocates for subsidized share car parking, competitive pricing for suburban parking, to make driving less appealing. The implementation of high-speed bus systems makes it easier to get around with a personal vehicle. The risk of autonomous-induced sprawl, Amin argues, makes priorities like these more important. The conversation has just begin, she says, but it’s crucial that these things are worked out before, and not after, the self-driving cars show up.