responsibility

Can you become "not responsible" simply by putting up a sign saying that you are not???
I saw a big cement truck--a cab with two long mixers linked behind it--with this sign on the back: "Not responsible for broken or cracked windshields. Stay back 200 feet."
First of all, a person couldn't read the sign more than 1-2 car lengths back, let alone 200 feet. And second, does putting up the sign really take away the responsibility?
If so, here are my signs:
Not responsible for forgetting to pick up the milk.
Not responsible for saying what I think without worrying about how you feel.
Not responsible for giving poor feedback on your writing.
Not responsible for coming to class with an ineffective lesson plan.

Of course I am responsible! For those things and for a host of others. And a sign doesn't take it away.