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 Hawaii, no trespassing signs serve as a clear indication that entry onto the property without permission is not allowed. Under Hawaii Revised Statutes Section 708-815, a person commits the offense of simple trespass if they knowingly enter or remain unlawfully on premises after reasonable warning or request to leave by the owner or lessee of the premises or other authorized person. The warning can be verbal or by posting signs at reasonable intervals along the boundary of the land. The signs must be sufficiently legible and placed in a manner that any person approaching the property can be reasonably expected to see them. Regarding liability, Hawaii follows the general rule that landowners owe a lower duty of care to trespassers than to invitees or licensees. The presence of no trespassing signs may help to establish that a person was on the property unlawfully, potentially reducing the landowner's liability in the event of an injury. However, Hawaii also recognizes exceptions where a higher duty of care is owed, such as in cases involving attractive nuisances that might draw children onto the property.