Objective criteria for identifying risk of collision are an indispensable prerequisite for the safe conduct of vessels and for efficient navigational assistance i. I N T R O D U C T I O N . Vessel traffic service systems (VTS systems) have been established, throughout the world, in areas of high traffic density. These systems, operating with such tools as VHF radiotelephony, land-based radar installations and ship's data processing, gather information to respond to collision and grounding danger, to organize traffic flow and assist allied activities. It is expected that VTS systems should effectively reduce the number of collisions in the VTS area. But efforts for collision avoidance can only be successful if the VTS operator in front of the shoreside radar screen is precisely instructed as to which measurements he has to make to identify a risk-of-collision situation. Uncertainties about the direction in which ships must take evasive action arise in VTS areas, where ships are free to engage in encounters.Collision avoidance rules for sea-going ships cover two different principles, one applying to ' fairways', and one applying to the ' open sea'. For both environments, unequivocal and easy-to-follow rules must exist to determine risk of collision, and in which direction evasive action must be taken. As far as headon situations in fairways are concerned, Rule 9 of the International Regulations for Preventing Collisions at Sea meets these requirements:A vessel proceeding along the course of a narrow channel or fairway shall keep as near to the outer limit of the channel or fairway which lies on her starboard side as is safe and practicable (Rule 9 a). This wording does not describe a collision situation. However, it is obvious that there does exist risk of collision between vessels proceeding along a fairway in opposite directions, when either of these vessels claims the space to the lefthand side of the fairway centre-line. There is then a definite instruction to take evasive action towards the right-hand edge of the fairway. Distances between vessels, time to closest approach, as well as distances to the fairway centre-line or the fairway edge can be measured and may, therefore, be used for the identification of a risk-of-collision situation. There is not much room for doubts as to what VTS operators should do. They have to verify each vessel's compliance with the obligation to keep clear of the fairway centre-line in the face of oncoming traffic. o 2. UNCERTAINTY AT SEA. When a ship is heading for the open sea, having left the fairway's lateral buoyage, the one-and-only ' Rule 9' must give way to as many as nine other rules, some of them imprecise and liable to various interpretations. Despite the uncertainty and diversity of these rules there is one 389
