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 Iowa, the use of 'No Trespassing' signs is recognized as a method for property owners to warn individuals against unauthorized entry onto their land. According to Iowa Code section 716.7, a person commits trespassing when they enter or remain on property without the right or permission to do so. The presence of 'No Trespassing' signs serves as a clear notice that entry is not permitted, which can establish that a person has knowingly trespassed if they ignore the signs. This can lead to civil and criminal liability for the trespasser. Furthermore, Iowa law generally imposes a lower duty of care to trespassers compared to invitees or licensees. Therefore, if a trespasser is injured on the property, the '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. It is important for property owners to ensure that their signs are visible and placed at regular intervals to effectively communicate the private nature of the property.