Dano, Creative Commons, Flickr
You can stay in the worst fleabag motel with dripping taps and electric baseboard heaters, but it will have a "save our planet" sign when all they really care about is saving on their laundry bill.
Yet in fact they work, and can be made to work even better. According to the New York Times, the Journal of Consumer Research examined how often people follow the requests of such cards, and why. It also found that we are sheep. 37.2% of guests "help save the environment" by returning their towels to the rack with the traditional sign. But when the sign said ""75 percent of the guests who stayed in this room had reused their towels" the rate went up to 49.3%.