Am I Driving Safely? signs

During today's morning commute, I saw a bumper sticker that had a variation of the "Am I Driving Safely?" message. As I read the unique ID for that particular truck, I realized that I have never seen any of these vehicles drive erratically. It makes me wonder: do the signs act as a deterrent for the drivers to practice defensive driving or do they act as assurance for the rest of the people on the road? Hmm.....