This report describes an experiment to determine the effect of fog on the speed and spacing of traffic on the M4 motorway. A discussion of visual range in day and night time fog is included to show that these two conditions should be considered separately. In the range of day-light fogs tested (visibility down to 150m) the relations between mean traffic speed and visibility are well defined and remain substantially unaffected until the driver's visibility distance, as given by a measure of the meteorological visual range, falls below about 200m. In a similar range of night-time fogs mean traffic speed are more variable, but the trend is similar to day-time if the driver's visibility distance is taken as the maximum distance at which a vehicle rear light of about 2 candela intensity can be seen through a dipped headlamp beam. In conditions where the visibility distance does not fall below about 150m, average traffic speeds are generally sufficiently low to enable most drivers to stop within their visibility distance, but the reduction in speed with reducing visibility is accompanied by an increase in close following causing an overall increase in risk. Around one-third of all vehicles follow within a 2 second inter-vehicle time gap when driver visibility distance are reduced to 150m in day or night time fog conditions