Definition: Regulatory Signs
The term regulatory sign describes a range of signs that are used to indicate or reinforce traffic laws, regulations or requirements which apply either at all times or at specified times or places upon a street or highway, the disregard of which may constitute a violation, or signs in general that regulate public behavior in places open to the public.
So, it would seem that a frost law sign would in fact be a regulatory sign. This is a small but important piece of technical information.