While driving through South Dakota, I was amazed by the number of billboards along the side of the road (90 West) for Wall Drug, the everything-you-could-ever-want destination in Wall, SD. Upon crossing the eastern border of the state into Sioux Falls, we immediately began seeing the signs. Now, note that Wall is about 300 miles west of Sioux Falls, so that's getting the word out early. They also get the word out often, because the billboards continue all the way to Wall, even when there is no other advertising - or literally anything else - along the side of the road.
I was only able to take a few pictures, but check out Flickr for photos of some of the billboards - they are really great, and can convince even the most schedule-dedicated road-tripper (that was me) that a stop at Wall Drug is a good idea.
