It is not easy to compare as there are lots of confounding variables - self-driving is not activated at random or all the time, but typically on highways, which

It is not easy to compare as there are lots of confounding variables - self-driving is not activated at random or all the time, but typically on highways, which are less accident-prone. They are also deactivated in difficult conditions such as bad weather which are also hard for human drivers. You can imagine a future with all cars are equipped with a self-driving system that always "passes the buck" to a human when conditions degrade - of course the system will have less accidents than humans! The statistics will even show human drivers being worse than before the advent of self-driving!