When must a vessel display running lights during the day?

A vessel must display running lights during the day in restricted visibility to ensure that it is visible to other vessels and to avert potential collisions. Restricted visibility can be caused by conditions such as fog, rain, snow, or heavy seas, where the range of sight is significantly reduced. The requirement for running lights in such conditions is part of international and inland navigation rules, which aim to promote safety on the water by making vessels more visible in circumstances where other vessels may not be able to see them clearly.

In contrast, running lights are not necessarily required in good visibility conditions, including open seas, unless there is a specific navigation requirement. For docked operations, standard practice also does not require running lights as the vessel is stationary and within a controlled environment, typically surrounded by other structures. Thus, displaying running lights in restricted visibility is vital for ensuring safety and compliance with maritime regulations.