Human beings drive close to 100 million miles between fatal crashes, so it will take hundreds of millions of driverless miles for 100 percent certainty on this question.
One thing about this "100 million miles": It includes people who are not allowed to drive. For example, because they are drunk. The number for non-drunk humans is even better and it would be a more fair comparison. We exclude robo-drivers who are not allowed to drive as well.