Do I have to call the police if I hit a traffic sign or guard rail?

Yes.  There are several hit-and-run laws in Virginia. This is one of them. If you hit unattended property, you are required to find the owner or, if not possible, leave a note with your name and details. Then, you are also required to report it to police within 24 hours.  Virginia Code Section 46.2-896 sets forth the requirements for drivers involved in such an accident.