What must vessels with operational radar consider regarding safe speed?

Vessels with operational radar need to consider the effect of sea and weather conditions on radar performance when determining a safe speed. This is crucial because various environmental factors can impact radar efficacy, such as rain, fog, and sea state. Poor visibility due to these conditions can lead to radar obstruction or false readings, which in turn can affect a captain's ability to make informed decisions while navigating.

When vessels operate in conditions that may degrade radar performance, it’s essential for the operator to adjust speed accordingly. A slower speed allows for more time to interpret radar returns accurately and enhances reaction time to potential hazards. This consideration is vital for collision avoidance and ensuring overall navigation safety.

The other options, such as the vessel's location, distance traveled, or speed of nearby vessels, are certainly important factors in navigation and safe operation but do not directly address how radar effectiveness is impacted by environmental conditions, which is critical in maintaining safety while navigating with radar assistance. Thus, focusing on the environmental impacts on radar leads to a better understanding of safe speed requirements in practical maritime operations.