An Unwelcome Curiosity, For Far Too Long!

Does anyone know what purpose, if any, the small, blank, red square signs below many of our DEAD END signs serve?  Are the red signs simply meant to draw attention to the yellow signs above?

And when these curious - and possibly useless - signs are covered in graffiti for years, what purpose does that serve?  Who benefits from a street sign covered in graffiti for years?  Would the city ignore a STOP sign if it were covered in graffiti for years, too?  

Of course, one can be arrested by HPD for creating graffiti, and I suppose the punishment would be worse if that graffiti is sprayed on a street sign.  "Graffiti is not good for the community," the judge proclaims to the perpetrator, "especially when it is sprayed on our street signs!"  Meanwhile, Robert Perry over at DPW says, "Graffiti on a street sign?  Just leave it!  I sure hope the judge sent the perpetrator to jail where he belongs!"