"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?"
I have only experienced it once in about a year living in the Colorado mountains. The rest of my life has been spent between 1,000' and MSL. Have never seen such dense fog at lower elevations.
Perhaps it was a combo of altitude, temp, and up-sloping pressure driven convection. Not sure. But not even the birds were flying, saw several magpies walking around somewhat bewildered.