When are vessels considered to be in "restricted visibility"?

Vessels are considered to be in "restricted visibility" when they cannot see each other by eye. This definition is crucial in maritime navigation as it signifies conditions that could impede a vessel's ability to safely maneuver and avoid collisions. The COLREGs emphasize the importance of situational awareness, particularly in conditions such as fog, rain, or darkness, where visibility is limited.

In situations of restricted visibility, vessels must take appropriate measures, such as reducing speed, sounding appropriate signals, and assessing their surroundings using radar or other navigational aids, since relying solely on visual sighting is not possible. This underscores the need for vigilance and adherence to specific navigational rules designed to enhance safety under such conditions.

The other options do not accurately represent the definition of "restricted visibility." High speeds can lead to reduced reaction time but do not inherently define visibility issues. Navigating in shallow waters and the presence of many vessels could pose different navigational challenges that are not synonymous with the limitation of visibility in the context of the COLREGs.