Drop plans for adding road and improve whats already there

You announce to add road to the game but struggle to add propper curves.
Why not go a different way and optimize the pathfinding engine to prefer paved undergrounds (already existing in game). so one can make their own roads.
Currently concurrent traffic is no problem. If it's kind of a goal to add collision boxes to the vehicles, than it might be an idea to add some kind of one-way-markings and lane separatings to the game, so one can make his own roads.
So you nee no fancy extra tiles but have maximum flexibility.