"Picking up your litter puts road-workers at risk" — is this strangely-worded road sign grammatically correct?

Yesterday I came across a road-sign (just coming onto the M40 at the Oxford services, if you're interested!) that seemed to read rather strangely. It read:

Picking up your litter puts road-workers at risk.

I think I can work out what it is meant to mean ("Don't drop litter on the motorway so other people have to pick it up- it's dangerous!"), but probably only because I had a fair idea in advance what it was likely to mean.

It's hard to put my finger on exactly what is wrong with it. Do others agree that it is phrased oddly? Is it grammatically correct? What would be a clearer and equally succinct way to say this.

Update: Just noticed this article about this sign (and others). Seems like I'm not the only one who finds the sign confusing.

Driver Rob Davis says... "In future, I won't pick up my litter. I certainly wouldn't want to put any workers' lives in danger by doing so."

Yes, but it's not really good English.

A participial phrasal noun ("picking up your litter") has been used as the subject of the main verb ("puts"). That's fine, grammatically.

The problem is that the phrasal noun is ambiguous, because it does not specify who would be doing the picking up. This is particularly bad for cases like this, where people are driving past the sign, and should not be taking the time to work out the exact meaning.

To remove the ambiguity, you can:

• Name the actor: You picking up your litter / Road workers picking up your litter / The queen of Sheba picking up your litter. These forms are awkward.
• Remove the genitive: Picking up litter puts... . This form removes the It's your fault! implication.
• Scrapping the whole sentence and starting again, but this time, saying precisely what you mean, using clear, unambiguous phrasing; e.g.: If you drop litter, people risk their lives picking it up.

Here's my amateur theory of what's going on here.

All our lives we are told to pick up your litter, put it in the bin, don't be a litter-bug, littering is a sin. You, you, you.

And then this sign goes and reverses the usual pattern (of making you the subject) and addresses you indirectly through an appeal based on the risk to road-workers. Instead of you, it's them.

So while technically it is comprehensible, it is awkward since nearly everyone will see the word litter and anticipate the familiar pattern of admonishment, which is why nearly everyone (who cares) will experience a double-take.

They could have made this less surprising by putting the word litter last, as in

Road workers are put at risk when they pick up your litter

When I first read it, I thought it refers to "you" as the subject who is "picking up the litter". Then I realized it refers to the "road workers".

I think it is grammatically correct but is not written with the subject first and that causes slight confusion.

I'm surprised the authorities thought this is an effective way to get the message across, rather than directly say

"Don't Litter ! It puts road workers at risk"