When is "risk of collision" considered to exist?

"Risk of collision" is considered to exist when there is any doubt about whether a risk is present. This definition is rooted in the emphasis placed by the International Regulations for Preventing Collisions at Sea (COLREGs) on the need for mariners to maintain vigilance and be cautious in their assessments of potential collisions.

When there is uncertainty, it's essential for a vessel's operator to take proactive measures to avoid a potential collision, rather than waiting for the situation to escalate. This precautionary approach helps ensure safety at sea, as conditions can change rapidly, and visibility or environmental factors may obscure a clear understanding of the situation. In many cases, what may appear as a safe distance can quickly change, making it crucial to err on the side of caution.

The other options outline scenarios that do not capture the broader and more precautionary definition of "risk of collision." For example, vessels can be moving in the same direction yet still pose a risk, and visibility of lights alone does not necessarily signify that a risk of collision is present. Therefore, recognizing any doubt as a potential risk aligns with the best practices for navigation safety.