It's dangerous riding a bike in the city—crazy drivers swerving in and out can seriously hurt you. So the next time a crazy jerk cuts you off and almost kills you, throw this magnetic yellow card at his car.
It's a warning that I imagine to be fairly effective. If I saw this on my car, I'd definitely try to be more cautious of cyclists while driving. Well, unless that stupid cyclist was a complete tool bag that was hogging up the entire road. There's a friggin' bike lane for a reason you idiot! Then, it'd just infuriate me.