IDOT has a program called the Yellow DOT that provides first responders at crash scenes with critical medical information to improve emergency care for persons involved in vehicle crashes.
Program participants are given a yellow dot sticker to place on the rear window of their vehicle. A personal information card that includes emergency contact information, medical conditions, allergies, and other important information, is placed in the glove compartment. When first responders arrive at a crash scene and see the yellow dot, they know to retrieve the personal identification card from the glove compartment. The program is a free service provided to all Illinois citizens.
I have not seen any cars with the yellow dot sticker, but now that I've learned of this program, I'll probably being seeing them everywhere.