You rarely see bumper stickers on expensive cars, or new cars of any sort. A bumper sticker on a Bentley is as out of place as a rattlesnake tattoo on a baby. And you rarely see bumper stickers on a car driven by a black, a Hispanic, or an Asian. It's very much a Caucasian thing.
Bumper stickers come in all varieties. Some advertise schools, or honor roll students. Some are humorous reactions to such stickers, such as "My child knocked up your honor roll student." You also see a fair number of stickers with the names of sports teams, or sometimes just sports. And sometimes a person wants to advertise his religion; the fish motif denoting Christianity is not uncommon.
But the most common bumper stickers are political ones. And those almost always express liberal/left viewpoints, often with lame puns. Examples:
"Dykes love Bush....Uh, not you George"
"No blood for oil"
"BUSH LIED. Thousands died."
"Bush and Dick. Four more years of getting screwed"
"Lick Bush. Kerry Edwards 2004."
"Doofuss" (with a picture of Bush)
"Gays against Bush"
"Why is there money for war but not for education?"
"You can't hug your child with nuclear arms"
"The religious right is fundamentally wrong"
"Straight but not narrow"
"Jail corporate crooks"
It's possible that part of the reason conservatives don't advertise their beliefs -- try counting the number of McCain/Palin vs. Obama/Biden stickers you see -- is that they are worried people who disagree might vandalize their car. (Liberals needn't worry because conservatives rarely consider vandalism a valid form of political expression.)
So why do people feel obliged to advertise their beliefs this way? It can't possibly be that they think that they're going to influence others by advertising their own beliefs. ("Well, I was going to vote for McCain, but ever since I saw that Obama/Biden bumper sticker on that car early this morning, I've decided to go the other way.")
For liberal Caucasians, it's all about showing the world what a good person you are.
When you think about it, every bumper sticker is a boast of some sort. "I brake for animals" is not an effective way to prevent tailgating; it's merely a way of advertising your moral goodness. Putting a religious-themed sticker on connotes a moral goodness of a different sort. Sticking the name of a prestigious university on your rear window is a way of advertising your intellectual superiority. "Proud parent of an honor roll student" even admits to the deadly sin. A wiseass answer to any of these bumper stickers is a way of showing off one's "wit," or at least of passing off someone else's wit as one's own.
So why more liberal stickers than conservative ones? Could it be that liberals tend to be the types who like to boast of their moral superiority? While neither party has a hammerlock on smug self-righteousness, one of the teams seems to have a fairly firm hold on the lead.