You dont have to have a sign. If you are on or using property that does not belong to you, you are trespassing.
The sign is just so stupid people know that they will be prosecuted. Mailmen trespass every day, but the property owners dont fight it because he is doing a service for them.
"Trespass to Land"
In modern law the word trespass is used most commonly to describe the intentional and wrongful invasion of another's real property. An action for trespass can be maintained by the owner or anyone else who has a lawful right to occupy the real property, such as the owner of an apartment building, a tenant, or a member of the tenant's family. The action can be maintained against anyone who interferes with the right of ownership or possession, whether the invasion is by a person or by something that a person has set in motion. For example, a hunter who enters fields where hunting is forbidden is a trespasser, and so is a company that throws rocks onto neighboring land when it is blasting.
Every unlawful entry onto another's property is trespass, even if no harm is done to the property. A person who has a right to come onto the land may become a trespasser by committing wrongful acts after entry. For example, a mail carrier has a privilege to walk up the sidewalk at a private home but is not entitled to go through the front door. A person who enters property with permission but stays after he has been told to leave also commits a trespass. Moreover, an intruder cannot defend himself in a trespass action by showing that the plaintiff did not have a completely valid legal right to the property. The reason for all of these rules is that the action of trespass exists to prevent breaches of the peace by protecting the quiet possession of real property.
In a trespass action, the plaintiff does not have to show that the defendant intended to trespass but only that she intended to do whatever caused the trespass. It is no excuse that the trespasser mistakenly believed that she was not doing wrong or that she did not understand the wrong. A child can be a trespasser, as can a person who thought that she was on her own land.
