S.A.T.S

This device doesn't physically damage the vehicle or its occupant, and is therefore less illegal than an egg launcher.

Basically, it consists of a reservoir tank mounted to the underbody, with an airtight-sealed pop-up lid (solenoid actuated). Under the lid is a filter that keeps in the goods but lets out the smell. Inside the reservoir is a rather nasty concoction of spoiled milk, rotten eggs, aging fish oil, year-old cat urine (the urine's a year old, not the cat it came from), and fecal matter from various pets. Beneath the reservoir is a hot air vent, leading directly from the engine compartment.

When you decide the person following you is encroaching on your personal space, press the button labeled "SKUNK". This directs airflow from the engine to the SKUNK reservoir and lets it cook for a few seconds to build up pressure, then the lid pops open and lets the God-awful smell waft behind your vehicle, ensuring a sizeable gap between you and your aggressor.

(disclaimer: this might take a few minutes to work if the offender has a cold at the time, but it should clear up the sinuses before too long!)