This is a cop only achievement meaning that you must be playing as a cop to earn progress towards it and unlock it. If after finishing you still don't have it go to the tier one event and pick "thin blue finish line". Instead of racing just drive up to the AI In third place and keep bumping into him. Each time you do you should get a nudge (it shows up at the top of the screen). You can nudge from any side. The game is pretty lenient with it. I found it easiest to ram from directly behind. Just keep repeating and I'd recommend backing out to the menu each time you finish the race to make sure your progress saved.