This is a question that can be difficult to answer. First of all, the studies seem to disagree on the numbers in regards to whether it is usually the motorist or bicyclist. One study purports that motorists (auto, truck, bus, etc.) are almost always - at an alarming 90% - responsible for collisions with bicyclists. Yet, another study states the exact opposite: that 90% of auto v. bicycle crashes are the fault of the person riding the bicycle.
The bottom line is that both a California motorist and bicyclist carry the same duties to each other; namely they have to operate their mode of transportation carefully and reasonably. This means each have to watch out for the other. If a bicyclist is not properly in his her lane, that does not mean the motorist will be completely without fault if he or she did not act reasonably to the diversion. Sometimes, a 50/50 split in liability may be negotiated by both sides. Of course facts and evidence make all the difference.