You can use the 'enter' key to get a list of all nearby objects you can interact with, and this should allow you to use the signpost. Since not all signposts are at the edge of the street, there isn't necessarily an easy, effective way to solve this. I'm going to close this as a non-issue for now, and if I see more complaints revolving around it, we'll revisit it internally.
Apparently there's a fairly simple solution to this that we might be able to implement, so we'll look into doing that, regardless of what I said above.