"It looks ridiculous, therefore, probably isn't be correct." I'm reminded of something Shinichi Kudo once said, "Don't be tricked with what you see now..sometimes the most unimaginable thing..is the truth!" I'm still not sure if this is one of those circumstances, but you see my point.
Haha, I don't want to eliminate the ridiculous. But when it comes to grammar, long practice has taught me to trust my instincts. If you spend a lot of time reading, you develop a good sense of what's right and what's wrong. I wish I could think of how to look up this case in a grammar guide, but since I can't, my best guide is my gut.
Finally, the signs have words on them and the sentence is about what the words read as, and the sign also tells the person/people reading it what to do; that's dialogue, and justifies quotes. Yes, it's an inanimate object, but it's still barking orders.
I should have been clearer: I meant it's not dialogue because it's not written as dialogue. The sentence lacks a dialogue verb like "says" or "reads." You're right that there shouldn't be a comma after the word "from," because it's not a dialogue tag.