Previous TRL research had investigated the reasons for the change in trend for the number of car occupant deaths in the late 1990s. This report presents further research analysing data from the Co-operative Crash Injury Study (CCIS), one of Europe’s largest car occupant injury causation studies.
CCIS is now the most substantial source of information about seatbelt wearing in accidents in Great Britain. Seatbelt wearing rates were found to be lower among injured car occupants than in general traffic. The lowest rates were for the fatally injured: 80% of male drivers killed during the day were wearing a belt, and 66% at night. These rates did not change significantly between 1994 and 2004.
A statistical investigation of CCIS data developed the understanding of what causes accidents to be least survivable in terms of collision parameters, detecting casualty patterns at differing levels of survivability and presenting trends in un-survivable accidents. CCIS data were analysed using the statistical programme ‘AnswerTree’, which applies the Chi-Squared Automatic Interaction Detector technique. It emerged clearly that CDC extent (a measure of deformation) and ETS (Equivalent Test Speed) are key to the survivability of an accident. High values of these variables lead to high un-survivability.

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