Megan McArdle :

When it comes to the glorious new future of self-driving cars, I’m something of a bear.
 Not a radical skeptic, mind you: I think self-driving cars are coming, eventually. I just think that “eventually” might be further off than people suggest. Getting to “level-four” automation — where the car drives itself, all the time, with no human input — is still a formidable challenge. Getting to level-three automation, where the car mostly drives itself most of the time, seems imminent. But the actual advantages of level-three automation over cruise control and early warning systems seem dubious. If people are distracted when the car needs them to take over, then level-three automation could end up being more dangerous than the older systems. And if people aren’t distracted, which is to say they are grimly staring at the road with nothing to do for hours at a stretch, then it seems worse than actually driving the car.