From the California Vehicle code:

Spilling Loads on Highways

23114. (a) No vehicle shall be driven or moved on any highway unless the vehicle is so constructed, covered, or loaded as to prevent any of its contents or load other than clear water or feathers from live birds from dropping, sifting, leaking, blowing, spilling, or otherwise escaping from the vehicle.



Ergo, clear water and feathers from live birds are the only thing that can legally fall off a truck in this state.