General conceptual considerations which apply to all occupant protection devices are examined formally to help clarify thinking about potential effectiveness of devices for which empirical estimates cannot be made due to the absence of any, or sufficient, field data. By considering hypothetical functions relating to the crash process, it is shown how overall field effectiveness of occupant protection devices depends on two elements. First, the specific dependence of effectiveness on severity in crashes, which flows from the engineering of the device and its relation to human biomechanics; this could, in principle, be determined in the laboratory. Second, the actual distribution of crashes by severity (and type) that occurs in real traffic; this cannot be determined in the laboratory. These considerations offer insights into why the actual effectiveness of occupant protection devices in preventing fatalities, as recently determined empirically from field data for car occupants (front and rear) and motorcycle riders, may be lower than expected based on laboratory tests.
Abstract