Equating bumper stickers with a warning label, the research of social psychologist William Szlemko indicates that people who apply the rearward-facing declarations to their cars are much more likely to use their cars to show rage on the road than people without such stickers, reports the Washington Post. The message of the bumper stickers themselves has no relevance to the result; peaceful messages of unity are just as much a warning sign as are offensive or hostile statements.
That is not to say that drivers without bumper stickers are rage-free behind the wheel, however. But they are less likely to use their car to show it to other road users, which means they are less likey to cause accidents as a result. Szlemko's psychological background leads him to conclude the reason for the sticker-related road rage is a fundamental human concept of territoriality.
Those more likely to personalize their cars with slogans are also more likely to engage in risky behavior because they are subconsciously protecting territory they believe is theirs - including the portion of road they are currently occupying.