Driving Message Board

If you are trying to communicate with someone in front of you while driving, you can honk, flash the bights, or wave.  However, there are limited options to communicate with people behind.  You know those programmable light up boards that scroll through schedules or announcements at airports or on signs outside department stores?  I would take one, attach it to the back window of my car, and have preprogrammed messages displayed by the push of a button.  Think of all the nice and pleasant things you could write to brighten the day of the driver behind you.