Fog impedes visibility and therefore the ability to react to changing conditions in time. This increases crash risk. Lower speeds in fog can only partially reduce the negative effects. The extent to which visibility is impeded is, of course, an important factor. In this regard, the Dutch national weather service KNMI states the following: "At a visibility of less than 400 metres, traffic may be affected. Dense fog with a visibility of less than 200 metres is dangerous. Very dense fog with a visibility of less than 50 metres forces people to drive at walking pace" .
Research on the road safety effects of fog is generally somewhat older and, to our knowledge, a recent review article is lacking. A US study on the effect of fog on highway crashes in Florida  shows that the effect of fog (visibility less than 1 mile/1.6 km) on crash risk is greatest near entry and exit ramps and in the leftmost lane. The study also showed that, compared to clear visibility, fog results in lower speeds and higher vehicle density (5-minute averages). However, a simulator study  showed that the speed reduction is often insufficient to react timely and appropriately to changes in the road environment, speed reduction of a vehicle in front or a suddenly crossing pedestrian. The study also showed that professional drivers maintained a lower speed in fog conditions than non-professional drivers. Another simulator study  compared experienced and inexperienced drivers and found that both groups drove equally fast in fog. However, in conditions of clear visibility, experienced drivers drove faster, meaning that experienced drivers showed greater speed reduction in fog than inexperienced drivers. Fog also adversely affects staying on course, a Naturalistic Driving study  found.