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 Illinois, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Illinois law recognizes that trespassing is both a civil and criminal offense. Under the Illinois Criminal Code (720 ILCS 5/21-3), criminal trespass occurs when a person knowingly enters or remains on the property of another without authorization. The presence of 'no trespassing' signs can serve as clear notice that entry is not permitted, which is important for establishing the 'knowingly' aspect of the offense. Additionally, Illinois follows the general rule that landowners owe a lower duty of care to trespassers compared to invitees or licensees. Posting 'no trespassing' signs can help reinforce this limited duty of care and potentially strengthen a landowner's defense in the event a trespasser is injured on the property and attempts to claim liability. However, the specific protections and implications of 'no trespassing' signs can vary, and it is advisable for property owners to consult with an attorney to understand the full extent of their rights and responsibilities under Illinois law.