Is there a way to define rodas in unity for AI movement?

I have a huge map (terrain) that have roads. Is there a way in unity to make AI walk from one point to the other through this roads? I know the existence of navmesh, but it is generated for all the map. I couldn’t find a way to define it in the roads.

Not sure how you built your AI, how about creating several gameobjects connected in an one-dimensional array and moving the AI along this way?