I guess I'd check to see if it's legal in your state. I'm willing to bet it's illegal in Virginia, for instance, because everything is illegal here (don't tell the cops, I have neons inside my car ).

I'm also a bit confused as to how someone who cuts you off will be able to see the frown on the back of your car

But I like the idea. A smiley face would beat the usual little wave I give to kind people who let me over.