Remember those bumper stickers that said "I Brake for Animals?" etc., etc. You do? That's the point! If your custom logo was on a bumper sticker right now, some motorist would be reading it and thinking about your business!
Bumper stickers are cheap to produce, last a long time, and are almost always in front of someone. That's why bumper stickers never go out of style as a reliable and affordable custom promotional product.