This is a complicated question and it depends on a lot of factors like if it's part of a "no shooting" zone in the county, the type of road and if you're hunting or not. The definition of LOC is also dependent on these factors. For example, if you're hunting in a National Forest and driving on an unimproved road (i.e. dirt road, for lack of a better example), you can drive with the firearm "loaded" as long as there is not a round chambered. However, once you're on an improved road, you have to revert to what we commonly understand as UOC in CA. If you are hunting and have a round chambered, the game warden automatically assumes that you are intending to hunt/shoot from the vehicle, which is illegal.
It's complicated, but they teach you most of this stuff in the CA Hunter's Education course, which is required to obtain a hunting license (at least the did when I was 14...which was a long time ago
).