This year has been full of big news about the progress of self-driving cars. They are currently street legal in three states and Google says that on a given day, they have a dozen autonomous cars on the road. This August, they passed 300,000 driver-hours. In Spain this summer, Volvo drove a convoy of three cars through 200 kilometers of desert highway with just one driver and a police escort. Cadillac's newest models park themselves. The writing, one might think, is on the wall. But objects in the media may be farther off than they appear.

There's a wide gap between having a prototype and going to market, and it's particularly gaping for anything with a combustion engine. The law has a lot to say about cars, especially about who’s allowed to drive them, and answering all the legal questions could easily take the rest of the decade. For instance, when a self-driving car gets in a fender bender, who's liable for the damages? Should a computer choose to hit an animal or swerve off the road? How does the DMV give a robot an eye exam?

As the technical limitations fall away, these legal questions are becoming the self-driving car's biggest challenge. Unfortunately for Google, the solutions will have to come from lawyers and legislators rather than engineers.