International road regulations are valid in Bulgaria. Driving conditions in major cities, as well as on the highways, are fair to good. You might experience problems if trying to reach more remote areas because of the poor road conditions (read: potholes!) and the lack of proper signage.
Your home-country driving licence may be used for up to a year in Bulgaria, after which it has to either be replaced with a Bulgarian one or you have to re-take a driving examination to get a Bulgarian licence while keeping your foreign one. US driving licences are acceptable only if accompanied by an International Driver's License.
Some of the most common violations for which police in Bulgaria hand out fines include speed limit violations, driving without a seatbelt and driving intoxicated. You may also be curious to note that it is illegal to drive in high heels, open sandals and flip flops and to eat and drink behind the wheel. On most of the open roads around the country you will find speed traps. Frequently you will be warned by oncoming traffic flashing their lights at you, if not then you face a fine to be paid later at the local municipality.