Good point Ray. A few years back some kids thought it was funny to drive their truck around knocking over people's mail boxes. After replacing them a couple of times an older feller put his on a steel pole deep in the ground with concrete. The teens hit his mailbox and were severely hurt as the car came to an abrupt stop. He was sued and found negligent because the court found that he made that mailbox with the intent to stop it from being destroyed by the vandals and had created a hazard knowing that it would be purposely targeted. Crazy decision in my opinion but you have to be careful. If you landscaped your whole yard with boulders it would be pretty hard to claim the purpose was to prevent someone from driving through your house but if you strategically place them to prevent cars in addition to letting others know your intentions, you could end up in trouble.