- between sunset and sunrise - even before the sun goes down below the horizon, sometimes it can get pretty dark in some situations, heavily shaded areas. You're safer if you're seen.
- any unfavorable atmospheric condition; rain, fog, snow. One day I was out and there were snow squalls, I saw someone driving a white van with no headlights. A snowsquall hit and he disappeared withing 50 feet of me!
- any situation where you can't see beyond 1000 feet clearly. Even at 35mph you're covering a lot of ground.
All kidding aside, let's not meet by accident.