Want someone to stop tailgating you? Turn on your windshield wipers.
Careful with motorcyclists! My mom did this once and he drove up next to her and gave her a stiff middle finger. To her credit, she didn't intentionally do it -- she just wanted to clean her windshield.
More Autos & Vehicle Tips
When it's snowing, place a towel on the windshield so you don't have to scrape the snow/ice off.