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 Idaho, the use of 'no trespassing' signs is recognized as a legitimate way for property owners to notify individuals that they are entering private property and that unauthorized entry is not permitted. According to Idaho Code § 18-7008, a person commits the crime of trespassing if they enter or remain upon the property of another without permission when notice against trespass is given by posting or communication. The signs must be conspicuous and are particularly important on large or undeveloped tracts of land where boundaries may not be obvious. Regarding liability, Idaho follows the general rule that landowners owe a lower duty of care to trespassers than to invitees or licensees. Posting 'no trespassing' signs can help landowners reduce their liability in the event that a trespasser is injured on their property, as it demonstrates that the trespasser was on the property without consent and had been warned against entering.