A marine traffic accident may be defined in the wider sense as a collision or stranding and in a narrow sense as a collision. Several reports are available on the causes of collisions and on the relation of collision frequency to such factors as location, weather and time; S. Fujii continuing Asai's work has shown that the ratio, R, between the number of vessels in collision and the number of vessels registered increases with the length of the vessel between perpendiculars. Shiobara gives a linear correlation between log R and log G with a slope of 0·5 and a correlation factor of 0·95, where G is the gross tonnage. In other words the collision frequency is proportional to L1.4. He further shows that R is almost proportional to L.