Actually it's funny, you won't even NEED to do that. When I had my P71 people automatically got out of my way. And going on one lane roads and randomly tapping your brakes as a car passes by will almost always show them braking assuming you're radaring them. Oh and let's not forget my favorite "pacing" them. I've actually had this one guy HIT A CURB because I was behind him pacing him. I have to say it was quite a bit of fun screwing with people in that car.

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*edit*
Getting a rhino push bumper and getting a P71 with the door strobe light in it will easily pass for being an unmarked police unit. Mine had (illegally actually) the darkest tint possible. You couldn't see inside the car if you put your face on the outside of the glass.