I saw this sign on the train, and wondered what the effect is. The trains in Boston aren't particularly dirty, they're clean most of the time in fact. From time to time, there are newspapers or Dunkin' Donuts coffee cups or bags and other random items.
My hypothesis, is that most people keep the train clean, not because it is their tax dollars, but because they have a personal belief of the need to do so. These people were brought up to pick up after themselves. From time to time, accidents are bound to happen, a person might forget a coffee cup like they might forget a glove or a phone on the seat. Given that I rarely see trash on the T (the Boston train system), there are a few reasons for this. One, as already said, people have a personal desire to keep the area clean. Two, people sometimes leave trash, and it is an honest accident. Three, people carelessly leave trash. The division between the a careless person and an honest accident is unknown to me, and I don't have much basis for deciding what the percentage of each might be.
Back to the sign. Who is it targeting? Is it thanking people who would have already picked up after themselves? Is it reminding people who are on the fence about whether or not to wait until the train station to dispose of something? Is it aimed at blatant litterers who might consider otherwise upon reading that their tax money is at work to pick up after them?
I don't think the last thought is the purpose of this sign. Any one who sees this sign, might see the tax dollars as money they've already spent. They're already getting taxed in many different forms, who is to say which one goes towards cleaning up the T, or how much. What is the chance that a publicly maintained clean T will result in lower taxes? (This was one of the immediate thoughts that was in my head when I first saw this sign)
I think there are opportunities to have a more directed and interesting sign in place of this one. It could impart a similar message, in a different form. One possibility is the exclusion of the sign all together.