Reduced speed limit precedes the fog bank, so yes, it’s reduced injuries and fatalities considerably. It serves as a warning system, seeing the speed limit drop and if it drops a lot, you know it’s bad before you’re in the thick of it, can’t even see the lines on the road and can’t decide whether to keep moving so you won’t get hit, or stop so you won’t hit somebody.
Zero visibility in a matter of seconds does not compute with most people.