“And why does the game keeps changing a large country road to a large street and starts building houses next to it.”
I have observed that building railroad tracks over the roads helps to prevent this issue.
Just build a small section of rail track over the road, close to the cities, and they will not be able to convert the road to street any more (they can convert sections that are close to the city, but they can’t jump a rail track while converting roads). If the city is spreading out, you have to build tracks on every place they can connect to your country road (if the rail track is surpassed by the city they can convert the road again).
For me it’s working. Someone else has tried it?