PREDICTING SEATBELT USE IN FATAL CRASHES FROM SEATBELT OBSERVATION SURVEYS

There is a large difference between the rates of observed seat belt use by the general public and belt use by motor vehicle (MV) occupants who are fatally injured in crashes. Seat belt use rates of fatally injured occupants, as reported in the Fatality Analysis Reporting System (FARS), are much lower than the use rates found in observation surveys conducted by the states. An important question is suggested by the data: Is there a way to predict the FARS use rates based on observed use rates? Development of a mathematical model that describes a functional relationship between these rates might provide some theoretical and practical insights concerning seat belt use and the prevention of fatal injuries in MV crashes. The relationship between FARS and observed rates was explored by using two initial assumptions: (a) belt users and nonusers are equally likely to be involved in fatal (PFCs), and (b) belts are 50% effective in preventing deaths in a PFC. A PFC was defined as any collision with sufficiently severe impact forces to kill a non-belted vehicle occupant. These assumptions can be represented as a simple mathematical relationship between the use rates of fatally injured occupants and the observed rates: F = (1-E)*S/((1-S)+(1-E)*S), where F is the FARS rate for each state, E is the effectiveness of belts in preventing fatalities, and S is the observed rate for each state. The fit of this model and the data was examined by comparing each state's actual FARS use rate with the rate predicted by the model. It was found that the model does not fit the state data points. Next, the effects of changing the initial assumptions in the model was examined. Changing the seat belt effectiveness parameter could not provide a good fit without using an unrealistic assumption, i.e., that seat belts are 71% effective in preventing fatalities. The inclusion of a risk coefficient for nonbelted occupants provided a reasonable fit of the model to the individual state data points. The major findings of the study were: (1) that a simple, straightforward mathematical description of the expected rate of seat belt use by occupants killed in MV collisions does not fit the FARS data, and (2) that a model consistent with the data can be obtained by incorporating the assumption that non-users of seat belts have a higher risk of involvement in potentially fatal collisions than do seat belt users. These findings suggest that a primary enforcement seat belt law is an appropriate countermeasure to target the high-risk driver population.