This paper discusses the use of rates to evaluate the effectiveness of countermeasures and to compare the safety of different countries and of different periods of time. The use of a rate requires an understanding of the shape of the curve relating the two variables. The use of the rates 'death per registered vehicle' or 'death per vehicle-kilometre' does not provide a consistent measure across time when there is a nonlinear relationship between number of deaths and number of vehicles. Predictive equations with one variable forming both part of the independent and dependent variable will suffer from spurious correlation. Smeed's equation does not predict the number ofdeaths in the original data set with any degree of overall accuracy, and cannot be assumed to predict accurately the number of deaths in other countries. The use of the number of persons killed rather than the number of fatal accidents might be questioned as one is the outcome of the other. More subdivisions of the total would be of moreinterest such as the numbers of different road user types within age groups. While deaths might be one of the common currencies of roadinjuries for comparing countries, there is little excuse for using them for examining trends within one country. Some performance measures that use the number of accidents and the number of persons (injured and uninjured) that are involved in them would be more meaningful than using deaths per vehicle. (A) This paper was published in a special issue of Accident Analysis and Prevention entitled 'Theoretical models for traffic safety' and for the covering abstract see IRRD846002.
Abstract