This 3 feet visibility in fog thing.
Setting aside pollutants, there is surely a theoretical minimum vis. due to water droplets in suspension [or slowly descending] but I cannot dig one out.
I write as a professional meteorologist [retired] who lived well before the UK clean air act, and who spent many many hours on RAF airfields all over.
Paying due regard to the definition of Met. Vis., I can honestly say that I have never experienced anything within touching distance of 3 feet [shall we call that one metre?]. I suppose my personal minimum might be 10 metres?