I utilize my windshield washer to keep tailgaters at bay. Its amazing that if you push the button or pull the lever a lot of windshield solvent will come out, go over your car, and onto the car behind you.

It works well for me.

Sometimes when I ride my motorcycle, people will do it to me even if you arent tailgating them. You don't know if they are just stupid or trying to be a-holes.
Not to mention dodging cigarette butts flying out side windows.