I took a defensive driver's course some years ago. Something interesting I learned is that even on the most beautiful, sunny day without a cloud in the sky you still can't really see an oncoming car at about 1 to 2 miles out. They showed us a few slides and then a video clip of an oncoming car with and without headlights on, on a very long, very straight stretch of highway. Even from a couple miles out in those pristine conditions, you could just barely see them but with their headlights on you could very easily spot them. Ever since then it's habit for me to always have mine own.