California Vehicle Code 21211 tells us that cars (and humans, bicycles and other objects) may not interfere with the reasonable movement of bicyclists taveling in bikeways or bicycle paths.
From CA CVC 21211:
(a) No person may stop, stand, sit, or loiter upon any class I bikeway, as defined in subdivision (a) of Section 890.4 of the Streets and Highways Code, or any other public or private bicycle path or trail, if the stopping, standing, sitting, or loitering impedes or blocks the normal and reasonable movement of any bicyclist.
(b) No person may place or park any bicycle, vehicle, or any other object upon any bikeway or bicycle path or trail, as specified in subdivision (a), which impedes or blocks the normal and reasonable movement of any bicyclist unless the placement or parking is necessary for safe operation or is otherwise in compliance with the law.
Some exclusions include utility vehicles, garbage trucks and tow trucks engaged in an activity such as trash removal.