> But if visibility is good, I don't see it making a difference much in what speed you're going.

Problem is, the speed limit is usually set with local factors in mind. I.e. if the speed limit doesn't make sense to you then the people who set the limit likely knew something you didn't.

Try driving in Finland, Sweden or Norway at dusk in the fall. That kind of attitude will often result in a white-tail or a moose through the front window. :D