On a freeway, every one travels at high speed - when the lead car has to slow down due to heavy fog, other drivers behind the lead car may not see the slow down in time and they crash leading to a chain of car crashes.
Police and insurance companies generally hold a pileup created due to sudden fog an act caused by God since there is no way to determine legal fault.
Never trust that the person in front of you won't just suddenly cram on their brakes to avoid a squirrel. Their tires might violently fail, you never know. If you run into something in front of you that didn't run onto the road laterally, then that means you are following too closely.
If something nails you, that is another story.