A few side effects, actually. On a larger scale, a car’s emissions affect the health of the planet. And on a smaller scale, its specs and features are directly linked to public safety. A driver in the market for a new car might want to know, for example, if she’s going to be sharing roadways with more SUVs than Smart Cars. “If I buy a 4,000-pound car that can accelerate from zero to 60 in three seconds, I have a side effect on a person who buys a 2,000-pound car that accelerates in 14 seconds,” Burns said. “I call that ‘second-hand physics.’ I think at some point people are going to start asking these second-hand physics questions.”

Until people know more specifically what Apple and Uber are trying to build, it’s hard to know what to ask first. Even the broadest cultural and ethical questions about the potential social impact of self-driving vehicles are just taking shape. The engineers building these vehicles don’t necessarily have the answers. But other major shifts in transportation provide something of a blueprint for what to expect.

“One of the things that’s true about driverless cars, and it’s been true for railroads and automobiles, is there tends to be this popular mistake to say, ‘We have this piece of technology,’ and utterly detach it from the infrastructure that’s necessary to make it work,” said Richard White, a professor of American history at Stanford University. “The infrastructure has always been the critical thing.”

He doesn’t just mean technologically, although that’s certainly the case with brightly painted lines on highways, well-maintained streets, satellites linked to GPS, and other environmental features necessary for self-driving functionality. Key infrastructure also happens to be largely publicly funded. But the transportation systems that rely on public infrastructure don’t serve everyone equally.

“Google’s out there inventing this wonderful car but everything operates on this public infrastructure that the public pays for,” White said. “So Google gets this massive private profit, while the public will not [uniformly] get the benefit of the technology, and while still paying for the infrastructure that makes it work. That’s how it is going to go.”

“I’m not a technophobe, and I can see all kinds of benefits,” he added. “But these are complicated problems. A lot of this is: Who is going to pay for it, and who gets the benefit from it?”

Perhaps Apple and Uber are savvy to avoid such questions for the time being. They are businesses focused on making a profit, after all. Many advocates for self-driving cars say there’s opportunity for the technology to help everyone, regardless of socioeconomic status, by driving down car ownership and encouraging municipal fleets, perhaps as a replacement for aging and inefficient bus systems. If that is indeed what lies ahead, we won’t know until someone builds a self-driving car that cities can actually buy.