In the U.S., each state has different regulations governing the use of stickers on car windows. AdvertisementAdvertisementAccording to psychologist William Szlemko and a team of other researchers at Colorado State University, there's definitely such a thing as too many bumper stickers. That's because a study they conducted in 2008 found that the quantity of stickers on a car was a predictor of road rage. Bumper stickers represent a mainstream, and sometimes humorous, way for people to express themselves on every subject under the sun. But bumper stickers featuring profanity, a protected form of speech, occasionally lead to a citation or arrest.