The cause of an increasing number of ‘radar-assisted collisions’ is the inadequate link between the radar receiver and the human operator. In contrast to all other traffic situations, the seaborne radar display forces the navigator to assess an imminent danger of collision from a ‘bird's-eye view’. When navigating in reduced visibility those vessels are most dangerous which are detected on the radar screen as being on a reciprocal or nearly reciprocal course.

It is normally no problem for the navigator to assess the collision threat from a vessel he detects optically as being near his own course line; the heading of the opponent clearly indicates the situation. A head-on situation indicates the necessity to take evasive action and the aspect of the opponent's port (or starboard) bow is left (right) indication for a left (right) change of bearing. The collision danger arising from any other target located in good visibility, and not near one's own heading line, can be assessed by repeatedly measuring or judging the target's bearing. Thus two different principles are applied to determine the threat of collision:

(i) With direct vision, by assessing the movements of approaching vessels in a horizontal direction,

(ii) With radar, by assessing the future passing distances from the plan view of the traffic situation.