The methodology is definitely flawed, but how many people have actually driven in a majority of the cities in the list (and done so more than once, and actually covered a significant chunk of the metro area) to have a decent frame of reference?
That's not to mention that the majority of humans are going to have similar driving habits going from place to place (at least in the US), and so the delta isn't going to be so huge that you're going to be able to definitively say that people from one place or another are 100% better at driving across the board. Wouldn't surprise me at all if the difference between the 3rd-place on the list and, say, the 60th were too small for most folks to say they definitely noticed a difference.
That being said, I would surmise that people with bigger egos are most certainly going to be more dangerous drivers, so it is no surprise at all that California places so high on the list of worst drivers. The less value people place in the safety and lives of other human beings, the less likely they'll prioritize driving safely over getting to their destination (ever-so-slightly) faster.