Sign Says---Please tell me if you know.

    It seem stupid.  To me, it does.  But I know there is usually a sound reasoning behind most things; it just escapes me.  But here is an example of something which makes me wonder every time I see it.  What am I missing?
   Driving home today. I was behind a truck for 6 or 7 miles.  A large orange sign on the back of the truck read  "CONSTRUCTION VEHICLE.  DO NOT FOLLOW"   Now I can figure out that the driver of that truck does not want you to follow him(or her)  onto a construction site.  Most people would not knowingly do so.  But at what point does the sign come into play?  I followed the truck (disregarding the words on the sign) until the truck made a left turn into Fane's construction site.  The sign did not light up or change position: it took up a large part of the back area on the truck anyway.  It didn't appear the sign could be turned or made to go out of sight.  That would make sense, if the sign were visible only when the truck was entering  a construction site.  But no, the sign is fully visible for miles.  Therefore, drivers are inured to it; it means nothing.
     Who decided on the signage, and how is it supposed to work anyway?