But the inconsistency is the problem.
Take a look at the top signs about driving on the shoulder. Who is to say there aren't twenty more variations?
Maybe a couple of
"Driving on shoulder mandatory"
"Trucks on shoulder only"
"Passing on shoulder permitted"
"$100 fine for driving on shoulder"
"Keep shoulder clear"
"No trucks allowed on shoulder"
There doesn't seem to be any standard. It's just text.