[allenp]

If you have a heightmap, the roads should be fairly easy. Pick start and end points (cities?) and use a pathfinding method like A* to plot the road. If you want something more stylized, you could select a handful of points from that method and then use beizer curves or something between them.

[wlievens]

And finally, add noise. Adding noise using midpoint displacement for instance greatly increases believability for very little effort. But you need to tune things carefully.