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 New York, property owners can use 'No Trespassing' signs to deter unauthorized entry and limit liability. Under New York Penal Law Section 140.05, a person is guilty of trespass when they knowingly enter or remain unlawfully in or upon premises without license or privilege. Posting 'No Trespassing' signs clearly communicates that entry is not permitted, which can establish that a person is on the property unlawfully. This signage can be particularly important for large or undeveloped tracts of land where boundaries may not be obvious. Regarding liability, New York follows the general rule that landowners owe a lower duty of care to trespassers than to invitees or licensees. By posting 'No Trespassing' signs, landowners may strengthen their defense against liability claims if a trespasser is injured on the property, as it helps to demonstrate that the trespasser was on the property without permission and that the landowner took reasonable steps to warn against trespassing.