You're not allowed to built a tile of road in an AI's turf, which is fine as the AI would presumably end up paying for it, but if you give a 'road to' order then your worker will road right across another players land (open borders assumed).
In the attached save you can see a Siamese worker roading up an AI's terrain outside Singapore.
You can road in an allied City State's terrain which I guess is deliberate.
In the attached save you can see a Siamese worker roading up an AI's terrain outside Singapore.
You can road in an allied City State's terrain which I guess is deliberate.