I 've created a straight, one-way road and tried to upgraded it to a highway. Although many “parts” where upgrading fine, some “parts” suddenly decided that they needed some curves 
so this is what happened
A closer look :
Where it should look like this :
I had to delete and rebuild the problematic road “parts”
Both roads, old and new, are one-way 3-lane roads
game version is 9.1.204
