Data gathered from the "robot" cars shows that they are safer and smoother when steering themselves than when a human takes the wheel. One of those analyses showed that when a human was behind the wheel, Google’s cars accelerated and braked significantly more sharply than they did when piloting themselves. Another showed that the cars’ software was much better at maintaining a safe distance from the vehicle ahead than the human drivers were.
That makes sense to me, as the driverless cars aren't in a hurry to get where they're going, or experience road rage.
Here's more on why we're inferior to robots from the MIT Technology Review.