The reason for the pileups is people travelling too fast and too close to the car in front for visibility conditions. In my opinion, if you can't stop in the distance you can see, you are travelling too fast. However, this means that in thick fog if you can clearly see the car ahead, you will likely not be able to stop if that car runs into a pileup. So once the first collision brings traffic to a stop, the rest of the scrum cannot help piling into it if all the drivers could see one another, which is the norm. Easily described, but not so easily avoided. This retired bugger just doesn't drive under those conditions, at least not on busy freeways. Sadly, commuters sometimes have to travel in fog, and on occasion even the best drivers can get trapped in a pileup due to the reaction time effects in a long lineup.