I wonder if the people who put bumper stickers on their cars would be willing to put the stickers on their shirts and walk into a crowded room. What if they had to voice their opinion face to face with real people who had real responses? Or, would they want to be defined by a pithy sarcastic phrase and risk the chance of being shunned by the crowd?
Slapping an opinion on a car doesn't allow for dialogue. In fact, I wonder if blogging has the same problem? Sure people can comment. And on the road people can cut people off or demonstrate their disapproval in other ways... but are these really the best ways of expressing opinions? In the car or on the web you're insulated from the response of others: you don't see the effect of your method in the face's of the people you influence.
When Jonathan and I moved to London, I noticed the lack of bumper stickers on people's cars. Then I came back to the U.S. and I was bombarded with attitudes staring back at me on the road and I couldn't ignore it. I think we should be more careful about sharing our opinions--especially with the method.