The principal aim of downsizing - reducing the mass and/or size of cars - is to improve fuel efficiency and reduce atmospheric pollution, but it has been argued that this would increase the number of casualties in car accidents. This report studies the relationship between car mass and risk of injury using British accident data for 1991-94, and the results are used to predict the effects of downsizing upon the number of casualties among car occupants and other road users involved in car accidents. It is recognised, however, that future development of car secondary safety through improved structural design and occupant restraint systems may significantly affect the relationship and hence the validity of the predictions. The report concludes that uniform downsizing - reducing all car masses by the same percentage - would lead to slightly fewer injured car occupants and non-car occupants, with the number of fatalities falling rather more (proportionately) than the total number of casualties. As far as the evidence examined in this study is able to indicate, fears that downsizing would adversely affect road safety appear to be unjustified. (A)

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