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 Alabama, the use of 'No Trespassing' signs is recognized as a method for property owners to warn individuals against unauthorized entry onto their land. According to Alabama Code Section 13A-7-1, criminal trespass occurs when a person knowingly enters or remains unlawfully on premises. Posting 'No Trespassing' signs clearly communicates that entry is not permitted, which can establish the 'knowingly' aspect of the trespass, making it easier to prove in a court of law. Furthermore, Alabama follows the common law tradition where the duty of care owed to a trespasser by a landowner is lower than that owed to an invitee or licensee. By posting such signs, property owners may reduce their liability in the event a trespasser is injured on their property, as it demonstrates that the trespasser was not invited and that the owner took reasonable steps to warn against trespassing.