Good question and I do not know of any official guidance on this. Assuming it is truly dark I teach my students to look for a road then try and land in the field next to it. There are a number of reasons this is a good idea-
1. Power lines are next to most roads and they are impossible to see after dark.
2. Emergency personnel can still get to you in a field.
3. Cars will not hit you, and you will not hit them in the field.
4. If it is a small road, it can remain open and not have to close due to an airplane blocking it.
5. If the landing is going to be a sloppy one, dirt should be somewhat easier on you and the airplane. I don't think this is true of higher gross weight aircraft though, because they land too fast to take advantage of it.
I think if a road is sufficiently large and not too crowded, a well-lighted 4-lane highway for example, it would be better to land on the highway with the flow of traffic.