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Factors that affect the injury severity experienced by occupants involved in road accidents include: accident dynamics, seat belt use, seating position, vehicle size, vehicle condition, driver condition, and driver action.

In a study by Nassar et al (International Journal of Impact Engineering, Vol 15, No. 6, 1994), the likelihood of occupant injury severity, for a myriad of risk factors, was estimated for three accident situations: single-vehicle accidents, two-vehicle accidents, and multi-vehicle accidents. The model was based on a classical dose-response formulation. Important risk factors in the model were accident dynamics, seat belt use, seating position, and vehicle condition. Probably important risk factors were vehicle mass, driver condition, and driver action.

Effectiveness of Seat Belt Use

The model was used to estimate the effectiveness of the use of seat belts in Ontario while controlling risk factors. The objective of this exercise was to assess the conditions where seat belt use is not effective in reducing fatalities and injuries from road accidents. A better understanding of this relationship should ameliorate some common misconceptions concerning the effectiveness of seat belts in reducing accident severities, and thus enhance compliance.

Estimates of the chances of death and major injury were obtained for Ontario road accidents using the models. Certain features in an accident were assigned as "initial" values, and changes from "initial" valuse were input into the model and the probabilities were estimated for each situation. For fatalities and major injuries, the probabilities were estimated for both front and rear seating configurations, with and without seat belt use. The results indicated that seat belt use is effective in reducing the incidence if fatalities and major injuries for road accident situations. The extent of their effectiveness, however, depends on accident characteristics.

The seat belt effectiveness results were compared to other studies. Evans, L. (General Motors Research Laboratories, Warren, Michigan, 1985) estimated a seat belt effectiveness of 30% + 8% for drivers in two car crashes using U.S. fatality data (FARS). For the Ontario data, an average seat belt effectiveness for fatalities was estimated as 33.3%. This value was arrived at by applying the estimated models to the actual conditions for individual 1986 fatalities and controlling for seat belt use (i.e. using the occupants' attributes and varying seat belt use to calculate the severity probability distribution). For single vehicle accidents, Evans estimated 62.2% + 5.2% for seat belt effectiveness in fatalities. The Nassar et al model estimated an average of 52.2% effectiveness for all fatalities.


In general, occupants in the rear seat had a lower chance of sustaining fatalities or major injuries than front seat occupants for front impact accidents. Nevertheless, when belted these same rear seat occupants could reduce their chances of death and injury by approximately 40% and 20%, respectively. The probability values for front seat passengers with seat belts were similar to the values for back seat passengers without seat belts, indicating that wearing a seat belt in the front seat, on average, was as effective as being an occupant in the back seat in fatal injury situations. This result was compatible with the results of Hobbs, C. and Mills, P. (Transport and Road Research Laboratory Report 1124, Department of Transport, Crowthorne, Berkshire, 1984).

This model is another valuable tool available at Walters Forensic Engineering to help assess injuries as they relate to the use of seat belts in vehicles.

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