I got in on this question really late.
In my city, a police officer is never off-duty. We are on-call 24/7.
To answer this question, I will give an example of a police officer off-duty. Generally, police officers do not like to make an arrest on their off time. Many reasons for this. He may be with his family and endanger them and being in street clothes does not mean much to most of the public. Its really a dangerous situation. But, it does and can happen.
Three off-duty detectives were having breakfast one morning at a restaurant that was in a small shopping mall. All the stores were on one level. An armed robbery was taking place two businesses down from the restaurant. A civillian came in the reastaurant that knew the officers and told them. The officers ran out the front door as the armed robber was also leaving the business he had just robbed. The officers ordered him to stop….police officers. The armed robbery turned around and fired one shot at them. The detectives opend fire and killed the robber. The shooting was declared a good police shooting, based on current rules and regulations of the department and by Tennessee State Law. This is why most officers, off-duty, plain clothes, and retired carry weapons.
I would not attempt to stop a traffic violator, off-duty, and in plain clothes. Its not worth it.
Off-duty officers always carry their badge and photo police identification. Once a police officer announces his authority and displays his police identification, this is all the law requires for an officer off-duty to make an arrest.
Any resistance to this arrest is assault and resisting arrest.
A police officer can make an arrest anytime, anywhere, anyday and for any reason, on-duty or off-duty.