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.
Step 2: Pick the location
Park yourself somewhere. Don't obstruct traffic or park anywhere illegal (median of the freeway), but park yourself on the side of a road where people tend to speed (which in most places is anywhere). High traffic areas will of course give you more opportunities to laugh. If you have a specific target in mind, you'll need to park somewhere they'll drive by.
Step 3: Let the fun begin!
Roll down one of the front windows, point the hair dryer out the window and enjoy. You can pivot the dryer to follow cars as they breeze by or lean out the window to make sure more cars see you. Either way, drivers will panic and slam on the brakes. If you're in a more obvious car (like a yellow beetle let's say) people will be even more confused and wonder what the local police department is thinking.