saw something similar once. 1st car pulled up at had a go at another driver who let rip in protest.
1st driver gets out and tells the other to do the same, which he did...squaring up for a kick off...1st driver whips out a warrant card and reads the riot act.
Off duty coppers are a lot braver than us mere mortals (I speak as the son of a London DI who learnt this lesson the hard way upon retirement)