Originally Met traffic officer. Now in general use. Alledgedly chosen as a motif because it's one of the only animals that'll actually eat it's own young! Until fairly recently a traffic officer could place a black rat sticker in their private car as an unobtrusive way of 'showing out' to colleagues, in the hope that they wouldn't get pulled for driving offences etc. Now-a-days it's more than likely that the car doing 90mph in front of you with a rodent sticker on it's number plate isn't actually being driven by a Black Rat, but a slag boy racer who's chancing his arm. Give him a tug.