In this example of what I am calling "co-billboarding," the Sports Illustrated Model is sucking a giant straw that links her billboard to the Dasani billboard. The campaign is genius execuation from Dasani and probably gets people talking about both ads. Somehow I think it's a better win for Dasani who otherwise receives less eye traffic.
Implications - What is being called "co-billboarding" is something that you don't see very often. Perhaps this is because most billboards are competing with each other to capture people's attention. Although marketing is a competition, using this type of advertising definitely makes both ads stand out more than they would each on their own.