Abstract
The frequency and duration of thick fog in Great Britain has been analyzed for the period 1958-1967. Generally, thick fog was found to be relatively infrequent, patchy, rarely widespread and of short duration. Its frequency inland has been falling in recent years, probably because of the Clean Air Act, 1956. The number of fatal and serious accidents and casualties were reduced but slight accidents and casualties increased significantly. Accidents in darkness and those involving pedestrians were reduced significantly whereas those involving more than two vehicles were increased significantly.