How is road completion determined?

I've been rebuilding roads since getting to Lake Knot City, and noticed that I seem to be the only one completing them. I built the first segment, and when I came back to the game a few days later, I noticed that other segments had appeared. However, since then, no other roads have appeared, although some paving machines have had partial materials loaded into them.

To me, this indicates that road completion (past the first few segments?) isn't actually shared between players, since it seems very unlikely that no other players would have completed later roads. Furthermore, the materials in the paving stations aren't always in units of 40 or 50, and as far as I know, that's the most granular you can get with the materials you have in-game.

Are materials or road completion actually shared with other online players?