Who's at fault if they suddenly open their door and I hit it?
You should read Vehicle Code Section 22517 which says " No person shall open the door of a vehicle on the side available to moving traffic unless it is reasonably safe to do so and can be done without interfering with the movement of such traffic, nor shall any person leave a door open upon the side of a vehicle available to moving traffic for a period of time longer than necessary to load or unload passengers."
I suggest you speak with a licensed California attorney who has experience with these types of cases.
A person must open his door on the side of traffic only when it is safe to do so. However, if you were driving to close to the parked cars, you also could be at fault. So it depends on the facts of the case.
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If you were in the normal flow of traffic, and the other party opened their door while you passed causing it to collide with your car, then the other party is at fault. Let your insurance handle this--give them all the information you have to support what you were doing, and where you were at the time of the impact.