The main reason for London being called the "big smoke" stems from the days before laws governing air pollution, mainly from coal fires, which gave rise to severe fogs, known as pea soupers or smog, particularly around November.
I visited relatives in London only a couple of times during severe smog, visibility was typically under 10'/3M which made driving almost impossible. You may be able to make out a dim glow from a street lamp on the other side of the road. Buses would proceed with the conductor walking alongside the front where he could make out the pavement/sidewalk and direct the driver verbally! Crossing the road on foot was a bit of a gamble, you needed a very good sense of direction to keep a straight line from one side to the other with no visual references, however there was unlikely to be any danger from the much reduced traffic, it couldn't move any faster than walking pace.