I remember this got posted to HN some time, and absolutely loved it - particularly as the sign was for all intents and purposes perfect and flew under the radar

I remember this got posted to HN some time, and absolutely loved it - particularly as the sign was for all intents and purposes perfect and flew under the radar even of the authority which would normally have been tasked with putting it up for months - heck, they even left it in place after having found out it was a guerilla piece! His observation that if you look the part, you can get away with just about anything matches my experience, though - when in university, the students' union I belonged to had all sorts of hi-vis vests and hard-hats which were used if we had planned any shenanigans - worked every time. You'll get farther with a hi-vis vest with a reasonable logo on it and a smile than with a smile alone. Bonus points if you also carry a clipboard and a VHF radio.