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 Colorado, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Colorado law recognizes the importance of such signs in establishing the boundaries of private property. Under Colorado Revised Statutes Title 18 Criminal Code, Section 18-4-504.5, a person commits third-degree criminal trespass if they unlawfully enter or remain on premises that are enclosed or fenced in a manner designed to exclude intruders or where notice against trespass is given by posting signs. The presence of 'no trespassing' signs can serve as this notice, making it clear that entry without permission is not allowed and may result in civil or criminal consequences. Additionally, Colorado 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 should a trespasser get injured on their property, as it helps demonstrate that the trespasser entered without permission and was adequately warned of the private nature of the property.