Methods of predicting the change in accident involvement of heavy trucks that would result from the mandatory installation of tractor-trailer front-axle brakes were identified. The choice of method for evaluating the front-axle brake issue became controversial as a result of the finding that a system-wide accident and exposure study would be prohibitively expensive and would take longer to complete than the life expectancy of the regulatory decision. The methodology known as "case-control" seemed an attractive alternative, but its previous use for heavy-truck accidents had been severely challenged. The usefulness of this method may have been obscured during a period of rightful questioning of the interpretation of the results from earlier case-control studies. A detailed analysis was made of the statistical limitations and practical feasibility of the case-control methodology. A computer simulation demonstrated that the method can be used to provide unbiased estimates of the coefficients in a logit-type causal accident model, and that only one control per accident is required. Further, it was recommended that rather than focusing on accident-involvement odds ratios, the model from the case-control methodology should be used in a probabilistic economic analysis to answer the regulatory question. Case-control was found to be a suitable approach for evaluating the front brake issue, but only at a level of threshold economic benefit and not in terms of absolute accident rate (number/veh-km). Moreover, it should be implemented only with certain safeguards, notably the validation of the randomness of control vehicle selection using classified vehicle counts. Estimated costs of implementation, although much below those of system-wide inspection surveys of truck exposure and accidents, were nevertheless substantial.
Abstract