Signs as moral agents. Or, sign as voice for the public. Either way, preventing the tragedy of the commons.
The expectation that users will follow signs; the likelihood of following a sign based on how much you agree/disagree with its tone.
Also: Look closely: User annotation of signage. “Take only what you need. To survive.” Signs as statements; the built environment modified to our ends. Hacked, even. How long until the built environment incorporates a comments feature? No need for Yelp! or UrbanSpoon; just call up the reviews of the restaurant in front of you.