Influence of light level on the incidence of road casualties and the predicted effect of changing 'summertime'

Previous studies of the apparent influence of daylight level and hour changes on the incidence of road casualties are reviewed and refined, by analysis of official databases for Great Britain (1969–1973 and 1985–1994) and the USA (1991–1995). New statistical methods, based on precisely computed altitudes of the sun for each accident location, are used to model casualty frequencies aggregated by week and hour of day, and locally evaluated associations between individual casualty incidence and solar altitude. Estimates of the altitude factor are interpreted causally to give counterfactual estimates of the effect of different clock time schedules on countrywide casualty numbers.