I'm going to patent this idea.

Neon signs in the back windows of cars that can be hooked up to a keyboard so that you can relay messages to your tailgating friend behind you ...some messages could be "go **** off and tailgate someone else" or "officer I'm speeding cause I need to take a dump". Lets not just stop with text you could have symbols like "the bird" or just a mad face depending on how pissed off you are.