Light Speed Constraint

> I like this last sign best:

After the previous two examples with their somewhat understandable context, the last one comes out of nowhere with no attribution or context. It's clearly a joke but I don't know whose or where they'd hang this. Unless it's original to the blog author, which is unclear to me.

I like the general thrust of the article, I just don't know where the last sign comes from and I'm confused.