Much of it has to do with pure geography, he explains. It's relatively easy to have self-driving cars navigate the relatively small and orderly world of a city, but it's another for them to venture across the country. They'd have to consider a much larger area with varying terrain and weather conditions.

We'd add that varying signs and rules of the road could complicate matters. And there's the question of whether or not you'd need a personal car -- if you can regularly summon a robotic ride within a few minutes, why buy your own and go through all the hassle of maintaining it? Companies like Lyft and Uber might be highly optimistic about a future of widespread autonomous ridesharing, but their vision has some basis in logic.