Picking up your litter puts road workers' lives at risk.This time, it's not ellipsis that causes the ambiguity, as it was in my previous post. Rather, it's a choice between two people who are doing the picking up:
You picking up your litter puts road workers' lives at risk.And also unlike yesterday, it's clear which one is meant here. There's not any obvious way that you picking up your own litter puts anyone else's life at risk; it's a good thing and you should do it. On the other hand, a road worker having to dash out into the traffic to pick up your crisp packet is a danger to that person.
Them [road workers] picking up your litter puts road workers' lives at risk.
This is another example of ambiguity in the reference of a pronoun, just like yesterday. But in this case, it's the invisible pronoun that linguists call 'PRO' (pronounced 'big pro', to distinguish it from 'little pro', which is a similar but different invisible pronoun). It's the subject of the clause picking up your litter, which has a gerund form of the verb in it (the -ing form). When you have that, you can have a subject which isn't pronounced (PRO) and gets its meaning (reference) from something else in the surrounding discourse context.
If something is the topic of the sentence, that's likely to be what's assumed to be the meaning of PRO, but other things are also relevant, like where the other possible referents are in the sentence, and what kinds of things they are (e.g. litter is not a likely candidate to be picking itself up here, for many reasons!). Shameless plug: you can read some actual research by me and my colleague Vikki (mostly her) here, on a related subject, and we cite a load of previous research that you can look up if you like.
In my sign, the road workers are the topic (well, their lives are, but you also need to be an animate creature to pick up litter). 'You', the reader, are also an option, because your litter is in there too. It's also closer in the sentence to the PRO that needs a meaning, which counts for something, but a strong combination of the road workers being both the topic and the more sensible meaning ensures that no one would read it and think that PRO meant you. Except me. I thought that. That's why I wrote this whole blog post about it.