Ah yes... but what if he tried to cross the 0/99 line while building the road? You'd lose track of the little feller for a few centuries as he reversed and built a road all the way around the world!
Good idea, it would be nice to have more control over the auto-engineer. I won't use him, I do everyone of my moves manually, right now I have 74 active engineers, because I don't trust them. (They build way too many railways) How about being able to assign an engineer to a city and script his moves? Road here, then here, then irrigate this square, then than one, then build a mine on that hill, then tranform those trees to grassland. 15 or 20 turns later, presto. Or at least have a user specified priority for what order they do things on what kinds of terrain?