How is road completion determined?

From my experience:

• 2 out of 40 total road sections were completed in my game by someone else - I don't know if they were actually completed by other players or not. This looks like tutorial thing to me.

• When building roads in areas with Chiral Network coverage they always had some resources added to construction, I only had to add some part to finish them. Looks like this works like any other structures from players.

• When building roads in areas without Chiral Network - 0 resources at start, I had to provide 100% resources to build them.

• Even after completing all roads I regularly see messages like "Player X added resources to auto-paver near Y" or similar wording.

To summarize I think that progress is shared but only to some extent (that explain why sometime you can see resources amount not dividable by 40 or 50), you have to finish most of the roads yourself. First few segments are exceptions, you won't have any roads finished next to Mountain Knot City.