Plate Flipper - License Plate Message Text
The Plate Flipper is a device that hooks to the license plate of your motorcycle, allowing it to flip down, and reveal a special message for...

https://www.bikernewsonline.com/2007/11/plate-flipper-license-plate-message.htm
Perfect for folks who ride congested roads and need to send out a "friendly reminder" to the cage behind them. You actually create any message you want.
I think what's needed is a programmable LED version, where at the push of a button you can display different messages.
I imagine it can also be used avoid paying tolls on toll roads, allowing you to evade the cameras.
Visit Plate Flipper online at...
http://www.plateflipper.com