It has been suggested that Single/Double Summer Time (SDST) should be adopted in the United Kingdom. Under this system of timekeeping, the sun would rise and set throughout the year one hour later by the clock than at present. This report considers the likely effect of this change upon the number of road accident casualties. Trigonometrical equations are used to calculate the altitude of the sun at any date and time for any point in the country. This allows the light level at the time of any accident to be represented by the altitude of the sun, and added to the basic accident data. Two alternative statistical models have been used to analyse accident data for Great Britain for periods between 1969 and 1994 to investigate the effect of darkness on the number of casualties. These show that darkness leads to more casualties, and that the effect increases with casualty severity. The study also analyses data for fatal accidents from USA for 1991-95. The adoption of SDST in Great Britain would transfer an hour of daylight from the morning, when there are relatively few casualties, to the afternoon and evening when there are more. It is predicted that this would reduce the number of people killed and injured in road accidents. The estimates of the reduction in the number of deaths per year range between 104 and 138, depending upon the assumptions made. (A)

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