2005 California Vehicle Code Sections 31500-31501 Article 3. Trailers

31500.  No vehicle upon which is loaded any trailer shall be driven
or moved on any highway unless the trailer is securely bound to the
vehicle in such a manner as to prevent the trailer from shifting,
toppling over, or otherwise becoming unstable.
31501.  Logging dollies being carried on trucks shall have at least
one set of tires resting against a steel cross rail or rails and
steel side chocks measuring not less than four inches from base to
tip which shall be securely attached to the truck in a manner
designed to prevent the forward and sideward movement of the dolly.