Vulnerable behavior is behavior which could result in an accident, and it is that element in the sequence of events leading up to an accident which police surveillance can affect. Since vulnerable behavior occurs more frequently than collisions, such behavior is easier to observe and lends itself to the statistical analysis of accidents. If the relationship between vulnerable behavior and collisions is established, the effect of police surveillance of accident prevention can be measured by observing the reduction in vulnerable behavior. Equations expressing this relationship by taking into account the probability that a particular type of collision may occur if a driver commits a certain kind of vulnerable behavior were developed. The equations were tested in a study of intersection accidents after the primary causes of such accidents were determined through an examination of traffic accident reports. Four forms of police surveillance were utilized: a policeman at a corner, two policemen on opposite corners, a policeman on a stand in the center, and two policemen on adjacent corners. As a result, the preventive effects of each form of surveillance on vulnerable behavior and various sorts of accidents were calculated. The vulnerable behavior concept may be adopted to other forms of accidents, including those involving pedestrians. The equations are explained in the text, and the results of the test study are presented in tabular and graphic form. An accident sequence diagram and 7 references are included.