The idea:
Use windshield wipers to spell out a message of thanks.
Attach several LED lights in a straight line on the car’s windshield wipers.
Simultaneously operating the wipers and LED lights leaves a word or pictorial afterimage.
When another driver yields to you during a traffic jam or lets you pass, you can create a message of thanks.
You can communicate your appreciation and add (optional) vocalization if you wish.
Your message can be created by a single touch from inside your vehicle.

Happy users: 
Both recipients and initiators of generous driving manners will be pleased.
This system offers a more appropriate form of appreciation than simply flashing one’s hazard lights.
The gadget can relieve stress during traffic jams, thus reducing accidents.

The creation has been tested in an actual vehicle.
The goal is to commercialize this as a marketable product.

Program source