When I tried to load the attached blend-file a few minutes after saving it, I got the message 'LIB ERROR: object lost from scene', and I realized that a tunnel I had made around the road was lost!
The mesh object that was lost was a cylinder named 'TUNNEL'. Like the other mesh objects ('ROAD', 'SIDEWALK LEFT' and 'SIDEWALK RIGHT') it had an array followed by a curve modifier, that made it follow the 'ALIGNMENT' path.
I checked the last Quit.blend, it had the same problem. Too, with the last autosave.
I am, once again, completely baffled. Can anyone make heads or tails out of it?