Try driving at 70mph on the motorway in 600m vis. It's not too clever.
Done that lots of time and it is easy with the required level of concentration. At 70 mph, things appear at 600 metres around 1/3 of a minute before you get there, probably 2-3 times the amount of time/distance needed to stop.
What is dangerous is patchy fog, when you go rapidly from 600 to 100m. 100m is less than many people's reaction time and car's braking performance.