The laws (as they relate to signage) concerning a private business owner relocating an improperly parked automobile vary widely. They don't just vary from state to state, but sometimes the vary from town to town. Some states require a certain height of letter. Some states require certain information be included on a no parking sign. Sometimes you have to include the phone number and fee for the towing company. I would suggest calling your city or state officials and ask them for a document stating the rules regarding a "legal" no parking sign for your location before you relocate any improperly parked vehicles.