Liz, this is a fascinating experiment. And an obvious logical extension of those animal and bird litter bins in school playgrounds.
The piano stairs video seems even more fun. Though I can imagine it becoming very irritating after a while. And driving people back onto the escalator.
But they're posing a fruitful question. How do we encourage and reward positive behaviour?. When it seems that most people only ask how we can catch and punish the rule-breakers.
When I was a child (less than three) I lived in Germany (dad in the RAF). My mum tells a story about the local park, and how we kids would literally feed the bin because it talked to us, saying "Danke" every time we put litter in it. If you watch you see parents encouraging kids to pick up litter, just like we did.