ROFL, this is too funny.
In all seriousness though, the range of where visitors will throw trash just needs to be tweaked a bit higher. They are actually pretty accurate at hitting them if one is close enough as long as they aren't angry. My zoo so far, as of year 29, has yet to have a litter issue and my solution wasn't as extreme as the OP picture. I just limited the size of paths to no more than 10m main pathways and used over-walks a meter or so off to the side over a lot of exhibits.
Then I made sure to put trash cans every 10-15m on at least one level of the walkway. Believe it or not, people will throw trash down or up a level to hit a bin on a different level of walkway. Other than that and keeping everyone happy, it's not been bad at all.