Essentially, time is how long it takes us to see.  Low visibility translates to a longer timeframe for the eye to discern details. Therefore, if any of the other three “factors of visibility” are hindered (i.e., a small size, poor contrast or low illumination) then the time needed to see properly is greatly increased. The time factor is especially important where motion is involved, as in driving.  Under low light levels, an object appears to move more slowly than under high levels of illumination.