No trespassing signs can be useful to property owners—especially on large tracts of land or land that is not developed—to put persons entering the property on notice that the property is private property and if they do not have permission to enter, they may be committing civil and criminal offenses related to trespassing. In some states notice is required for a trespasser to have civil or criminal liability for entering the property without permission.

No trespassing signs may also protect a landowner against liability to a trespasser, as a lower duty of care is owed to trespassers in some states, and putting a person entering the property on notice that they are trespassing may strengthen the property owner’s defense against any liability claims if the trespasser is injured on the property.

In Arizona, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Under Arizona law, specifically Arizona Revised Statutes (ARS) § 13-1502 to § 13-1504, trespassing can be a criminal offense. The presence of 'no trespassing' signs can help establish that individuals entering the property are doing so without permission, which is a necessary element for criminal trespass. The signs serve as a clear notice that the property is private, and entering without permission is not allowed. Additionally, Arizona follows the general legal principle that property owners owe a lower duty of care to trespassers than to invitees or licensees. By posting 'no trespassing' signs, property owners may strengthen their defense against liability claims if a trespasser is injured on the property, as the signs indicate that the trespasser did not have permission to be there and was warned of the private nature of the property.