Getting peoples attention to pull over?

I run across people all the time that need to be notified of something wrong with their vehicles, and have often wondered if there was some type of "app" for tablets that would let you scroll or flash a message to other drivers.

In the above example, my wife could key the message "check hitch" or "hitch connection" or any number of "flat tire", "antenna", "steps out" messages, and it would flash or scroll on the screen to alert the other driver.

Doug