I did this a few times in high school when I was bored. It simply consists of pretending to have a radar gun and laughing when the car speeding past you slams on the brakes. Even parked at the end of my driveway in the red minivan I drove, I had the neighbors slowing down upon seeing the hair dryer pointed out the window.
Step 1: Get a hair dryer and a vehicle
Any old blow dryer will work, although I suppose the larger and clunkier the better. If you don't have a blow dryer available, anything that looks like a radar gun will do (Pringles can maybe...or a painted Nerf gun). You also need a vehicle to sit in. A Crown Vic. is of course the most obvious choice, but as I said before I made this work with a red minivan.