The person backing out of the driveway.
if you're reversing into your driveway and someone hits you its their fault. you were moving away and they wern't paying attention. Now if you went back into the street to straighten out and back back in then it might be your fault, but if you were reversing the whole way and never came back out of your driveway it would be theirs
Both are equally at fault and you may both have to use your own insurance to pay the damage. If a police report was filed, then the police will determine the % of fault to each person. CORRECTION - Your INSURANCE COMPANY will determine the fault - not the police. Yes, it may be a 50/50 situation, unless one party admits 100% fault.
If you were fully in the lane, and he is backing from private property onto public property more than likely he will bare the majority of fault/liablilty.
What type of driveway? Was it a curving and could not see the car before rounding the curve? Is it straight; then why did you not see the car blocking the driveway before even getting into car and/or backing..
The backing car is at fault
If you reverse your car and then collide with a parked car , the person who did the reversing is at fault.
As a general principle, the person operating a moving piece of machinery is responsible for damage to any non-moving obstructions. Whether or not the other car was legally parked is irrelevant. It could be abandoned in the middle of the road and it would still be the drivers responsibility to see and avoid it.
Finding out the answer to these sorts of questions is what courts are for. The answer depends on the details, but from the brief description given, I'm inclined toward the position that it's probably your fault, or a mixture of fault, but with you being the one mostly in the wrong. People who are backing up are supposed to take extra caution.
It is not an offence to back out of a driveway, but it is an offence to back across a lane of traffic.
If you back into a car you are at fault, especially if the other vehicle was parked. You are the one with the responsibility to look behind you and to avoid the accident. If both cars are moving all bets are off.
The general rule of thumb is that any car moving that hits a parked car is at fault.
Don't force the wording of the event and I believe the answer to: "...when you back into someone....", is evident. Add to it, "...when exiting a private driveway...", where one is always required to yield to the traffic on the public street.