That's too idealistic. The reduction of accidents caused by human errors could very well be offset by a substantial increase in machine/OS/SW/HW caused accidents.
That's pretty unlikely though. It's not like cars are going from manual-everything to autonomous all at once. Multiple levels of traction control and steering assistance are already in control of your car, and they prevent more accidents than they cause. Autonomy is going to be rolled out in pieces and tested little by little.