One thousand seven hundred and eighty vehicles were photographed from the rear whilst being driven in fog on the m4. Speed, vehicle registration number, date and time were recorded on the photograph whilst separate equipment monitored the visibility level. The registered keepers of 600 of these vehicles were traced and sent a questionnaire asking for the drivers' opinions on the conditions in which the vehicle was photographed and on driving in fog generally. Seventy six per cent of questionnaires were returned. At a visibility of 50 m or less more than half the drivers were exceeding the speed at which they could stop within this visibility distance.

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