I'm glad for your same interesting with me about Highways!
Yes, Highways are really in the game (50% completed).

As I posted, the way to change the road_level 1 to 2 in export_descr_buildings.txt is unrealistic.

Maybe if we follow an "add new building" guide from Rome Total War to take an idea how to do this, is better.
I have already created and completed the new -in game- pictures - building cards for Highways!
For now remains the way to add the new building to the game...