Why is it so difficult to explain the decline in traffic fatalities?

In many highly motorised countries, the number of traffic fatalities has gone down by about 80 percent since the peak number, which was reached around 1970. What explains this decline? Is it principally the result of road safety policy, or have other factors made a larger contribution? This paper argues that it is difficult to give a scientifically rigorous explanation of the decline in traffic fatalities. There are five main problems: (1) There are very many potentially relevant explanatory variables. (2) Some of the relevant explanatory variables change slowly at an almost constant rate. (3) Data are incomplete or missing about many potentially relevant variables. (4) Some variables are affected by measurement errors or discontinuities in time series. (5) Many of the explanatory variables are very highly correlated with each other and with time. These problems are illustrated using Norway as an example. It is shown that the problems listed above can result in models that are non-sensical although they pass formal tests of model quality. The lesson is that one should never judge how good a model is merely in terms of formal criteria. Some strategies for developing more meaningful models are discussed.