In this advertisement, the Carnegie Melon Design Students are urging students to ride bikes not just for beneficial health, but also for environmental sustainability.  This reduces "one pound of CO2 for every mile you don't drive", according to the poster.  Visual rhetoric is the use of graphic design to appeal to cultural beliefs, but also as an act of persuasion to an audience.  Graphic design is useful because it helps manipulate an image to make it more striking to the audience, and that causes it to be more memorable so the message is more effective.