Sources of uncertainty in estimated benefits of road safety programmes

National road safety programmes have been developed in many motorised countries. Some of these programmes contain estimates of the safety benefits that were expected to be realised if the programmes were fully implemented. When these estimates are compared to actual outcomes, it is not uncommon to find large differences. This paper argues that differences between the predicted and actual results of road safety programmes could be the result of a large, but generally unrecognised, uncertainty inherent in estimates of the effects of such programmes. Ten sources of uncertainty are identified and briefly described. The possibility of describing these sources of uncertainty numerically, and of estimating their joint contribution, is discussed. It is concluded that at the current state of knowledge, it is not possible to meaningfully estimate the total uncertainty inherent in road safety programmes. The prospects of reducing uncertainty by means of research are discussed.