I think they're image files as textures... I could be wrong... but most I've seen are. You'd need to edit the texture itself I think. I'm prepared to be corrected on this though.
Not correct, if a route DLC already has the physical signs that deanmacgregor wants then he can just make new blueprints in a different folder with a new name and then reskin the texture of those signs to suit their needs.
That's what we're both saying here. The OP wanted to know if you can just "edit" a sign by typing a new name in some way.
Yes, I know, I was responding purely to the post I quoted and adding a bit of detail to how it needs to be achieved.
Not by default, you'd have to provide a host and download link for your new files, but so long as they lived alongside the originals you reskin, Workshop wouldn't block the route from upload.
If you upload the route and you include the sign assets in that route folder then yes, people would see those signs on your copy of said route. Other people can then also use those signs from your route assets in their own routes (and may ask your permission to publish them etc etc) Don't forget that if you publish a route then this will only have scenarios that you've posted for that specific copy of the route as the route folder will be different to any you may have based it on