Along Jack's comments - you need to understand how the law is enforced in the area. Some require the sign to be X high and spaced at Y distance and visible from Z distance and the like and along the entire perimeter of the property, while others only require posting at "access points". And like Jack said, folks will simply pull them down, when given the chance. Somewhere I recall seeing a sleeve that slid over T-posts - not exactly cheap. Some states also use the "purple" paint law as well.