I'm thinking we've got two different things here. I took Dub's description to be of a warning section at the edges of your safe driving area. I've mostly seen them on interstates or other limited access roads at the edges of the road; their purpose being, as Dub mentioned, to wake you up. They are more or less continuous. Rhuby's description I took to be of the discrete rows of the same type surface that are found on the approach to a tool booth or a work area and are even found on some secondary roads when an unexpected stop sign is looming ahead.