About cars, my experience (in the country/mountain) is :

• there are way way fewer cars at night; on small roads (small but not single lane), it is not unusual for me to ride 5 miles or 10 km and meet 0 or 1 car; of course it depends on the road (mostly), the day of the week and so on;

• there's something happening with night traffic: my theory is that the fact that drivers & riders need to lower their lights when they come across (or drive behind) someone else causes a greater concern for the other:

  • You acknowledge his presence.
  • You do something which is purely in his favour (lowering your lights doesn't help you to see better, it helps the other person).
  • You notice that the other driver acknowledged your presence and did something for you.

  This is more concern than most drivers ever show in day time, and according to my theory, it puts people into a different state of mind. Naturally you also occasionally encounter the guy who takes night for a chance to drive like a rally driver. Then you just hope that the presence on the road of a cyclist in the middle of the night is something that appears sufficiently suspect and incongruous to the driver's eyes, that it will exert extra care when closing in.

One other point to take into account is the drinking habits in the area: the guys who can get a bit hot in the late evening, and those who are wasted/tired when they drive home from parties and clubs between 4 and 6 AM. It also depends on the local driving habits: in the area where I live now, they really drive like shit in day time, in normal condition (they don't drive fast like where I grew up, they just do whatever & anything all the time). So it doesn't get much worse when they are drunk :-D

As far as I am concern, I am wary of dusk and dawn (and wait for the night to be almost pitch black to start), for at dusk/dawn there may be one of those few drivers who drive without a light, either because they forgot to turn them on, or because they believe they know better and don't need lights.

(In my answers in this thread, I often say 'I' and 'my' because I believe that one's experience can vary a lot depending on the area, the specific roads used, and one's riding habits and goals. For example, I have the feeling that English cyclists commonly ride on highways on which I'd refuse to put a tyre).