I suspect self-driving mode will be safer than human drivers only when every car is driving itself. Meat and silicon brains sharing the same road in large chunks of high velocity metal is a scary prospect.
I think it also would have been easier/faster to build networked vehicles that can communicate with one another over a standard protocol, but for some reason the industry didn’t opt to standardize things in that way, and now we have a bunch of rogue driverless cars that make independent decisions instead of taking into account each other’s plans.
You can call those driverless road lanes "tracks" and put some trains on it.
Or if your self-driving car struggles with tight corners, or complex junctions? You won't see many of those on the freeway.
On the other hand, if your self-driving car suffers random failures to stop, unexpected swerving or a propensity to hit into stationary objects - I agree lower speeds would be safer.