If you place a placard on the panel you may be advising that there is risk, but not limiting your liability. I believe it has been argued that if you have a "Beware of the Dog" notice on your property, then you are aware that the dog is dangerous and that INCREASES your Duty of Care!
The EXPERIMENTAL placard on some types is a warning notice of the passengers accepting the associated risk.
Lawyers being what they are, the defining factor in any argument will always be the equity and wealth (if any)held by the defendant, as long as that remains you'll always have litigation.