The textbook answer, the one you have to learn when you take drivers education around here at least, is that traffic should travel at a speed that is safe in the current situation (taking into account the environmental conditions, the condition of the road etc) but not exceeding the maximum velocity.

Poor visibility is one of the main reasons to reduce speed. As is things that affect handling, like snow or wet conditions.

So, “At what speed should a car travel at night in a 40mph zone?”

I don’t know. But if you almost ran someone over, you probably took a risk you shouldn’t have.