The objective of this report is to present a general methodology for determining the benefits (or disbenefits) of a change in vehicle design in terms of injuries prevented (or incurred) and lives saved (or lost) on the basis of the results of crash tests in which dummy responses are measured before and after the change is implemented. The information on dummy responses is typically available for several test crashes and contains specialized measurements such as maximum chest acceleration or arm bending moment during the crash. Using this information, engineering and biomechanical judgements may be formed as to the likely effects of the design change in the measured physical responses. The problem is to translate these assessments on the change in the physical response into changes in the numbers of injuries and fatalities expected to be observed on the road if the design change is implemented in the fleet of vehicles.

The solution is based on the observation that the probability of injury (of a given severity on the AIS scale) can be expressed in two ways: 1) as a function of the physical response measured in test crashes using the cadaver test crash data; and 2) as a function of a measure of crash severity such as speed at the time of impact (or delta-v) as recorded in a crash database. The idea is to relate the crash severity measure and the physical response level corresponding to it based on the probability of injury. Then a change in the physical response can be translated to a change in the crash severity measure and the new probabilities of injury can be determined. Given the numbers of motor vehicle occupants involved in crashes at various crash severity levels in the crash database, the information on new probabilities of injury at those crash severity levels is used to predict the numbers of injuries after the vehicle design change is implemented. By comparing the predicted numbers of injuries based on the probabilities of injury before and after the change is implemented, the effect of the change can now be assessed.

The calculation of the change in the number of fatalities is based on a relationship between the probability of fatality and the injury severities of a motor vehicle occupant. This relationship is established empirically using crash data. The predicted number of fatalities is calculated using the estimated probability of injury at various severity levels before and after the vehicle design change.

The original motivation for developing this methodology was the problem of estimating the effect of changing the Motor Vehicle Occupant Protection Standard No. 208 to allow the depowering of air bags. The methodology was then implemented to estimate the expected increases in the numbers of chest injuries and related fatalities as a result of depowering. A refined version of these calculations is presented in this report to illustrate the use of the methodology in a practical example. The numbers presented are based on the assumption of a 33 percent increase in chest acceleration across all crash severities and occupants. This assumption is not based on actual experience with air bag depowering and consequently the results represent a purely hypothetical case. However, apart from the assumption about the increase in chest acceleration, the calculations presented are based on actual cadaver data and crash data. They show how to establish a statistically significant increase in chest injuries when the actual information on chest acceleration change due to depowering is used.