INAL, but actually putting adversarial image attacks on real roads is already illegal. If you modify a street sign, and as a result someone dies, that's a fairly easy case of Involuntary manslaughter.

At a minimum, you can't modify street signs. Eg in Washington State:

  RCW 47.36.130
  Meddling with signs prohibited.
  No person shall without lawful authority attempt to or in fact _alter_, deface, injure, knock down, or remove any official traffic control signal, _traffic device_ or railroad sign or signal, or any inscription, shield, or insignia thereon, or any other part thereof.

(Underscore emphasis added). And if you're thinking about not putting it on a sign, but putting it elsewhere visible to cars:

  RCW 46.61.075.1
  Display of unauthorized signs, signals, or markings.
  No person shall place, maintain or display upon or in view of any highway any unauthorized sign, signal, _marking or device_ which purports to be or is an imitation of or resembles an official traffic-control device or railroad sign or signal, or _which attempts to direct the movement of traffic_, or which hides from view or interferes with the effectiveness of an official traffic-control device or any railroad sign or signal.  
  

Where I'm unsure is producing these with the intent or knowledge that they will/could be used by someone to go do this. None of this makes using these for research and experimentation illegal.