I saw this image today:
For the sake of this question, let's assume it happened in the front yard of the poster's house (so on his property)
If this really did happen, would this be considered a booby trap, because the nails served no purpose other than to destroy the property of people driving over the sign?
Would this trap be illegal, even though there's no way that it would reasonably hurt another human (if you're outside, you have shoes on, protecting you from the nails).