Abstract
The general accident reporting and recording situation is discussed and specific examples are given of the effects of different systems on aggregated and disaggregated accident data related to particular countermeasures. It is considered that the accidents being studied should be disaggregated into accident types to determine the real effect of a counter- measure. Accident type can be costed according to the average number of casualties and vehicles damaged. If this is done benefit /cost studies are meaningful since the analysis will be sensitive to the change wrought by the countermeasure.