The 4th Amendment Security Sign
A relatively recent Supreme Court decision (2013), Florida v. Jardines, contemplated whether police could walk up to a homeowner's front door with a drug-sniffing dog. Although the Court decided that it was, in fact, a search and that such action is impermissible, it compelled a debate as to what is and is not permissible.
Although the Supreme Court ruled in favor of the homeowner , other types of encounters may be deemed permissible. In fact, this past month, the U.S. Court of Appeals for the Tenth Circuit indicated that a simple "no trespassing" sign doesn't revoke implied permission for a police officer to knock on someone's door (or other types of encouters, such as solicitors). To revoke said permission would require something more.
Two law professors, Stephen E. Henderson and Andrew G. Ferguson, have created a website (FourthAmendmentSecurity.com) (in anticipation of a forthcomgin article on the matter, selling magnets, stickers and lawn signs that explicitly address the revocation of the implied consent to cross onto a homeowner's property.
Although legal articles typically reach only a small and particularlized audience, with this endeavor they hope to inform the public regarding what constitutional rights they have and raise awareness. Although it is essentially an attempt at humor, it does point out the often absurdity in the law. More information is avabilable on their website.