A joint Econometric Analysis of Seat Belt Use and Crash-Related Injury Severity Naveen Eluru
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Overall, the results indicate clear biases in the effects of variables on injury severity level when unobserved factors moderating the impact of variables is ignored and/or seat belt endogeneity is not considered.6. CONCLUSIONSThis paper formulates a comprehensive econometric structure that recognizes two important issues in safety analysis. First, the impact of a factor on injury severity may be moderated by various observed and unobserved variables specific to an individual or to a crash. Second, seat belt use is likely to be endogenous to injury severity. That is, it is possible that intrinsically unsafe drivers do not wear seat belts and are the ones likely to be involved in high injury severity crashes because of their unsafe driving habits. The structure of the model developed in the paper takes the form of a mixed joint binary logit-ordered response logit formulation that conveniently, and at once, considers all the issues of (1) systematic interaction effects among variables, (2) random unobserved effects in the influence of injury severity determinants, (3) potential endogeneity of seat belt use in modeling injury severity levels, and (4) random variations in seat belt use effectiveness. To our knowledge, this is the first instance of such a model formulation and application not only in the safety analysis literature, but in the econometrics literature in general. The empirical analysis is based on the 2003 General Estimates System (GES) data base. The focus in the analysis is exclusively on non-commercial driver seat belt use and crash-related injury. The analysis is also confined to the vast majority of crashes in which one or two vehicles are involved. Several types of variables are considered in the empirical analysis, including driver characteristics, vehicle characteristics, roadway design attributes, environmental factors, and crash characteristics. The empirical results indicate the important effects of all of the above types of variables on driver seat belt use and injury severity. In addition, the results reveal a substantial and significant negative error correlation between seat belt use propensity and injury severity propensity, which lends strong support for the selective recruitment (or sample selection) hypothesis. That is, safety conscious drivers are more likely to wear seat belts, and their defensive habits also lead to less severe injuries when they are involved in crashes. To summarize, ignoring the moderating impact of unobserved factors on the influence of injury severity determinants and/or the endogeneity of seat belt use in injury severity modeling leads to biased parameter estimates and elasticity effects. With respect to seat belt use specifically, our results suggest that seat belt usage and the safety-conscious driving attitudes of those who wear seat belts are about equally important in reducing the likelihood of a fatal injury (earlier research efforts do not disentangle these two different aspects of seat belt usage). Thus, from a policy standpoint, seat belt non-users should perhaps be subjected to both a fine (to increase the chances that they wear seat belts) as well as mandatory enrollment in a defensive driving course (to attempt to change their aggressive driving behaviors). ACKNOWLEDGMENTS The authors acknowledge the helpful comments of two reviewers on an earlier version of the paper. The authors would like to thank Lisa Macias for her help in formatting and typesetting the document. The second author would like to dedicate his part of the research efforts to his Father, Dr. Ramalinga Bhat, who passed away in May 2005. REFERENCESAbdel-Aty, M. A., C. L. Chen, and J.R. 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Kockelman (2005) Occupant Injury Severity using a Heteroscedastic Ordered Logit Model: Distinguishing the Effects of Vehicle Weight and Type. Presented at the 84th Annual Meeting of the Transportation Research Board and forthcoming in Transportation Research Record. LIST OF TABLES TABLE 1 Summary of Existing Discrete Choice Studies of Crash Injury Severity TABLE 2 Cross Tabulation of Injury Severity and Seat Belt Use TABLE 3 Estimates of the Seat Belt use Component of Joint Model TABLE 4 Injury Severity Component of Joint Model TABLE 5 Elasticity Effects for the Fatal Injury Category TABLE 1 Summary of Existing Discrete Choice Studies of Crash Injury Severity
TABLE 1 (cont.)
* The analysis is restricted to driver aged 65 and above. $ The analysis is confined to sports utility vehicles TABLE 2 Cross Tabulation of Injury Severity and Seat Belt Use
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