Are cars with bumper stickers dangerous?

Their drivers are.

Via Richard Wiseman’s excellent book 59 Seconds: Change Your Life in Under a Minute:

…people who personalize their car by adding bumper stickers or window stickers may be sending out powerful signals of territoriality, and they were curious to discover if having to share public roads with others could increase the chances of these driversâ experiencing road rage. To investigate, hundreds of participants were asked to report how many bumper and window stickers they had and also to rate their level of aggressive driving. The results revealed that drivers with more stickers admitted to driving more aggressively, including a greater frequency of tailgating and ramming.