the effect on accidents of the changes in light conditions in some hours at the beginning and end of british summer time has been studied. darkness is assumed to be the period between the beginning and end of lighting-up time. on the day bst ends, a period of one hour in the morning that was dark acquires a mixture of daylight and twilight and vice versa in the evening, the process being reversed in the spring. at least 5 weekdays before and after the change are required to yield sufficient accident data. for reasons explained in the report, the appropriate morning periods still provide too little accident data, and so have not been studied. great britain was divided into 6 regions to make some allowance for regional variations in lighting-up times, and accident totals were collected for each region for the relevant hour in the evening. accidents for the remainder of the day were used as 'controls'. some of the ameliorating effects of street lighting were avoided by only using accidents on roads with speed limits over 40 mile/h. the results were consistent over the country, and were in line with earlier findings. they indicated that the immediate effect of the clock change, outside built-up areas in the busy evening hours studied, was that in darkness the frequency of all injury accidents was about 50 per cent higher, and that of fatal and serious accidents was about 100 per cent higher, than in a mixture of daylight and twilight.(a)

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