The notion that all colleges would produce the (essentially) same result from this public billboard is absurd. At most, we can only speculate about trends. By it's nature, the billboard would be a dynamic thing, maintained by a rolling assemblage of predominantly 18 - 24 year-olds.

Many of the billboards would be ignored by almost everyone, and the groups that notice them would be more likely to persist if they had an actual message to get out. Certainly, there will be gratuitous use of genitalia that appears regularly, but not by an organized group whom maintain the policy of more cartoon penises on display in public.

I think there would be a huge artistic assault on the billboard from time to time, and a certain crowd will take offense to that kind of thing and deface it if they have an audience.

Placement is key, I think. If it's in front of the freshman dorms, then that's going to be a different billboard than the one outside the math department. There's no way to hit all demographics with one location at any reasonably sized university.