I find the safety obsession with self-driving cars to be amusing. Safety is the easy part. I'm sure they'll be able to make cars which drive themselves on highways and have as good or better safety records as humans in similar driving conditions. Programming them to not hit things and to avoid being hit by things is, I would guess, the simplest challenge right now.

I disagree.

Here's the scenario that convinces me otherwise:

You're driving on a residential street. It just happens that there's a birthday party in one of the houses ahead on the right. You can see a "HAPPY BIRTHDAY" banner in the front window. Cars are parked along the street, on both sides, in front of the house. Kids are playing with a ball in the front lawn.

Most human drivers will see the kids playing in the yard and will, at least a little, raise their guard. Kids, ball, street - balls have a tendency to go into streets. Maybe you slow down a bit, maybe not. But you're probably going to start paying close attention.

You keep driving. And now you're in front of the house driving between two rows of parked cars, slowed down a bit because it's narrow, plus, kids.

Suddenly, a ball appears from between two cars immediately ahead on your right, bounces once, then exits between two cars on the left. What do you do? You slow way down, or even stop, because you already know that:

1. there are kids playing in front of that house
2. with a ball
3. that ball just bounced across the street in front of you
4. kids follow balls

At 30mph, you need 45 feet to stop (not counting human reaction time, which adds another 45 feet). 45 feet is about three car lengths. I hope you had already slowed down a bit. If you didn't, you will never be able to stop in time.

In this scenario, knowing about kids and balls is what gets you to slow down enough to be able to stop in time. It's not enough to react when the ball appears. You have to actually understand the situation and prepare yourself for the worst. Dumb sensors can't do that. It takes some amount of intelligence. Would the computer in a self-driving car know how to identify "kids playing with a ball on a lawn"? Not today. In ten years? Maybe. With matter-of-life-or-death reliably? I wouldn't bet my life on it. Maybe in twenty years? But until they can, self-driving cars will have to be limited to driving impractically slow except on restricted-access highways.