In the near future you won't have driverless cars, both for technological and social/legal reasons, any commercial car will require a person in the driver's seat. That makes liability easy, whoever's in the driver's seat is responsible for the car and its actions. It's exactly the same as if you turn on cruise control in a "non-autonomous" car and rear end someone on the highway. You're liable, even though you're using technology.
The auto acceleration isn't really apples to apples with this. That's just a potential product failure (Like car's catching on fire when they get rear-ended). It doesn't really apply to autonomous cars more than non autonomous cars, except for the potential "autonomous cars are more complex", which is questionable anyway, considering how complex current cars are.