Deriving the correct costs for accidents of various severity levels by using unit costs per person-casualty class requires a knowledge of all the casualties of various classes within a specific severity level. If this data is not known the accident costs are underestimated. This paper shows the correct structuring of data to derive the costs of accidents of various severity levels. Different accident types have different average severities and differing numbers of vehicles involved. The evaluation of countermeasures must take cognizance of the effects on various accident types and thus in turn the different average unit costs of various accident types. For example, a fatal "right angle" accident has a greater cost than does the all-types fatal accident, while a fatal pedestrian accident has less cost. This paper also shows how the costs of accidents of various accident types may be derived. The accident data of the State of Victoria, Australia for 1978 are used to illustrate the procedures.