In my state in my home country, the police force is divided into traffic, patrol, crime, detective work and so on. Different roles require different qualifications and as such attract different people. Bribery is common. Detective work is the least lucrative and is a form of punishment for upstanding officers. The consequence of that is it’s the least corrupt of all the branches, it’s genuinely interesting work and the morale is high. Traffic is the most corrupt but it has the least consequence anyway.