Right now all the roads are just asphalt blocks, I was wondering if anyone has been able to figure out replacing roads with prefabs from a road set? (Guard rails, curbs, possible sewer systems, overpasses, etc.)
No, best you can do is change the material type for the road and change what blocks are used for road decorations, but you have no control over their placement.
Sit tight, although what guppy says is true, I've been exploring the possibility of adding some of the things you mention for some time now and have a couple of sets of prefabs you may find interesting.