How is a risk of collision determined according to navigation rules?

Determining the risk of collision is fundamental to safe navigation and is largely based on the concept of whether the relative position of another vessel is changing. If the compass bearing of another vessel remains constant, it indicates that the distance between the two vessels is likely not changing either, which suggests they are on a collision course. This principle reflects key navigational practices where maintaining observation of the bearing helps identify potential risks of collision.

In other options, while both vessels being on a collision course does address the possibility of collision, simply stating they are on a collision course does not emphasize the method of assessment through constant bearing. The speed of one vessel compared to another doesn’t inherently indicate a risk unless considered alongside their courses. Likewise, poor visibility may increase the risk of collision but does not directly determine it according to navigation rules; it complicates the situation rather than providing a clear metric for assessing collision risk. Therefore, the best determination comes from observing whether the bearing to another vessel remains unchanged.