I used to think that the most dangerous thing about bumper stickers was that they make curious drivers inch ever closer to the car in front of them in order to read the things ("He Put the Duh in W," perhaps, or "At Least the War on the Environment is Going Well," or "49% ***, 51% Sweetheart; Don't Push It," or "If There Is a Tourist Season, Why Can't We Shoot Them?"--for all of which I am indebted to http://www.bumperart.com). But no, bumper stickers pose another danger: drivers who plaster their vehicles with the things are more prone to road rage than drivers who leave their car or truck unadorned.
As scientists led by Lucy Troup and her student William Szlemko of Colorado State University report in the June issue of the Journal of Applied Social Psychology, it's a simple matter of territoriality. Researchers have long known that drivers who have a strong sense of personal space while in their vehicle are more likely to be road-ragers, and the more someone plasters his vehicle with bumper stickers and decals the more territorial he feels about the space inside.