Off duty cops are usually able to drive around with impunity - whether speeding or driving drunk - because 99.9% of the time, flashing a badge will immediately get you off the hook should you be pulled over.
If you were simply speeding, you'd get a wink and a nod and sent on your way. If you were drunk, the worse that would happen to you is that your cop "buddy" would drive you home and your car would get a free tow to the station and you'd be able to pick it up the next morning with no questions asked.
That is happening less and less thanks to PD dash cameras.
Now what happens is that the guy gets busted back a rank, if that, unless they end up making the news.
In that case, they get fired, appeal and come back after a prolonged paid vacation unless they are are they a “problem child.”
Yes, I have seen it happen.