from my experience, cops will most often not write other cops for minor infractions. but when it comes to more serious things like crazy speeding, careless or reckless driving, you will find more and more cops either writing them, or making a phone call to their superiors. also, you will find that for DUI, the cop's supervisor's will be called to the scene more and more, or just flat out arrested. these days,driver's liability is such a big factor in retaining a cop that there isn't much tolerance for driving issues.
cop culture is dynamic just like everything else, it changes over time. things are much different now than when i started, particularly in the Use of Force and so-called "professional courtesy" and the big "point five-oh", or 50% off meals and freebies... these sentimental traditions are falling by the wayside, often due to departmental rules and regulations, general orders or policy.