"Unexpected" quotation marks: Why?

I frequently run into uses of quotation marks on public display that simply baffle me. I have no idea who or what is being quoted, what the original reference is, or why it's necessary to indicate that this is a quote.

Clearly it's no accident: the quotation marks take effort to add. Someone had to go to the trouble of putting them there, someone who felt he had a reason to do so.

If it were a one-off occurrence, I would chalk it up to miscommunication. But this is so common a practice that I wonder if there's some common misconception, or perhaps convention in a foreign language which leads people to place quotation marks where they are not, in English, traditionally expected.

Alternately, is there some meaning communicated here that I am missing? Do these quotation marks actually indicate something of which I wasn't aware?

There are plenty of times that quotation marks can change the meaning of a short piece of text, however, I don't think that's the case here.

It's quite likely that this was indeed the result of a miss-communication, but not the kind you're thinking of. If you were received a written message telling you to place the words "road work ahead" on the sign, it might not immediately occur to you that the quotes were not intended to be part of the message.

This kind of thing is not that uncommon. Here's a more obvious example of what I mean:

Here, the intent was obviously to indicate that the message written on the cake should be with uppercase letters, but the baker mistook "(All Caps)" as part of the message itself.

You're not missing anything. There has recently been an unfortunate trend to use quotation marks where they're not warranted or appropriate. You can see many more examples of this in at the blog tchrist linked in a comment.

Quotation marks should not be used for emphasis or no reason at all, as in your example and most in the aforementioned blog.

While I find this practice (i.e., the mistaken use of quotation marks for emphasis) utterly deplorable in formal writing and hand-written signs, it might be somewhat excusable in electronic road signs, when more appropriate means of emphasis (e.g., italics, boldface, underline, etc.) are unavailable.

In CS (Sorry. That's jargon for "Computer Science!"), rendering a message, or even an ENTIRE sentence, in upper-case is considered gauche (The CS term is "shouting"); however, I believe its use for a SINGLE word or SHORT phrase is a reasonable substitute for the otherwise-unavailable tools of emphasis.