I call this complete baloney. I have seen absolutely no evidence that any significant proportion of the driving public even understands that slowing down will improve their fuel economy, let alone that they are actually doing it. In fact every day I am completely amazed by the number of people who barrel along at 85, irrespective of the type or size of car, apparently without knowing that because of their habits they are already paying $5.00 a gallon, and doing it voluntarily.
I know slowing down will improve my fuel economy, yet I choose to go a bit faster on the highways (when it's safe). I wouldn't have bought the car I did if I wanted to drive like grandma.