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 Oregon, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Oregon law requires that property owners provide notice to individuals that they are entering private property. Posting 'no trespassing' signs is a common method of providing such notice. Under ORS 105.700 to 105.710, if a property is properly posted with 'no trespassing' signs or the individual has been personally notified that they are not allowed to enter, entering or remaining on the property without permission may constitute criminal trespass, which can lead to civil and criminal penalties. Furthermore, Oregon follows the general principle that landowners owe a lower duty of care to trespassers than to invitees or licensees. Therefore, if a trespasser is injured on the property, the presence of 'no trespassing' signs may help the landowner demonstrate that they took reasonable steps to warn against entry, potentially reducing their liability for the trespasser's injuries.