I looked in to it a while back for here in Calif. It appears to be a Fed DOT standard. From talking to CHP, the Calif reg is Fed based.
The requirements specifically is that a vehicle, be it a car or tractor ect, be tied off at each corner. It can be chain/binder or ratchet strap. Each tie must be independant of the other. If the loader can lift 12' or more, it must also be restrained(this goes back to guys leaving the tractor running on the trailer, driving down the road and having the loader lever bouncing around, causing the loader to raise).
Here is the CHP email reply "The California Highway Patrol (CHP), Commercial Vehicle Section received your e-mail dated March 14, 2004, regarding the proper way to secure your small tractor to a trailer.
Section 29004 (a) (3) of the California Vehicle Code (VC) states in part, any vehicle transported on a conventional trailer shall be secured by at least four tiedown chains, straps, or an equivalent device. In addition, it is your responsibility to ensure the vehicle is safely secured for transportation on the highway. The type of securement device you use is only limited to its ability to safely secure the load. Therefore, a ratchet style cargo strap may be utilized to secure your load."
I had cross-referenced this to the Fed-DOT, but have lost the bookmarks.