An important element in the traffic safety decision framework is a prediction model that can interrelate the full range of alternative safety-system components and predict their performance quantitatively. Discussion focuses on the pre-accident stage of a preliminary model in which four phases are identified: (1) predisposition, concerned with the driver's biography and factors in his accident background; (2) initiation, concerned with the driver's vulnerability role, i.e., the expected number of hazards he incurs in a given mile of driving; (3) juxtaposition, prediction of the number of imminent collisions expected from the vulnerabilities; and (4) evasion. The last phase is analyzed with respect to confrontation and countermeasures to prevent collision. The vulnerability concept provides a useful and measurable index of driving errors, unsafe behavior, and impairment. Marked correlation has been shown between a driver's vulnerability rate and his accident record. 36 pp. Ref.