FOR traffic proceeding in random directions on a plane surface the frequency of collisions, if no avoiding action is taken, is approximately proportional to the square of the traffic density and directly proportional to the size and speed of the craft. Avoiding action is normally taken and the rate of collisions is therefore also governed by additional factors such as the visibility, the effectiveness of the collision-avoidance rules, the competence of personnel, the manoeuvrability of the craft and the efficiency of radar and other equipment.

In the post-war years there has been a considerable increase in the number of merchant ships. The number of vessels of ioo tons gross and upwards increased from less than 30,000 in 1948 to over 65,000 in 1976. When considering the population of ships at risk it is necessary to deduct special types of non-trading vessels and ships laid up. The number of trading ships in service has increased by approximately 100 per cent since 1950 (Fig. 1) and the greatest increase has been for large ships. The average length of trading ships has increased by approximately 40 per cent in the same period and there has also been an appreciable increase in the mean speed of ships.