Teade juhile
Send a short message to any car by its licence plate. Lights left on, a blocked driveway, or a thank you for letting someone merge. The owner claims their own plate and reads what was sent to that car.
What was hard: making it useless as a tool for harassment while still being useful for the ordinary case. Nobody is looked up in any register, a message is never public, and a sender can never tell whether a plate has an owner behind it, which is what stops people probing.