If a police officer violate a traffic law, who gonna write him a ticket?  

I think we common people have to complaint against him..But who dares or cares to do all that..Noone will be willing to follow all the procedures after registering the complaint.

Well, sadly, I doubt anyone is going to be able to do much of anything. Police officers have a bit more leeway in regards to bending the law, because of enforcing the law. Of course, that is abused some of the time but such is life and matters like that happen all of the time. If they cause an accident, then I am certain that they would blame it on the civilians driving around. Perhaps I am just the paranoid sort, but that could happen.

Civilians filing a report might work but it is a case of your word against theirs in this case. Let's face it, there could be some shady dealings going on. Not all police are bad, but there are some that give the one's that try to work within the law a bad name. Therefore, who knows if reporting them could do any good. I doubt most of the time it even will.

Other police officers, I have seen it done before.

And yes, complaints are taken very seriously I have seen it in the paper that police have lost their jobs from civilian complaints before.

Some officers do abuse their power though, I have to agree with that.