Other Visibility Problems
In city micro climates, there can be problems with visibility. But these are often related to pollution and smog rather than to fog or mist.

Smog in particular can be very dangerous as it can become so thick that is reduces visibility completely. In the UK, these are called 'pea soupers' as they are as thick as soup.

Classifying Fog
Visibility has to be less than 200 metres for motorists for it to be classed as fog. For aviation and in observations 1km is the upper threshold.