Rule 6

Safe speed

Any visibility

Proceed at a speed that lets you take effective avoiding action and stop in time for the conditions.

In plain language

There is no single number for a safe speed — it is whatever speed lets you avoid a collision and stop within a distance suited to the conditions right now. The rule lists the factors that set it: visibility, traffic density, your boat's stopping and turning ability, background lights at night, wind, sea, current, nearby hazards, and your draft against the available depth. Radar users get a second list — radar helps, but its limits count too.

Takeaway

Safe speed is the speed you can stop within — set by conditions, not by your schedule.