If two vertical red lights are present, where can vessels safely navigate?

When two vertical red lights are displayed, they indicate a specific navigational situation that is crucial for the safe movement of vessels. The correct answer, which is that vessels can safely navigate between the lights, directly relates to the meaning of this configuration.

In maritime navigation, two vertical red lights signify a specific channel or route that should be used by vessels. When navigating between the lights, mariners ensure that they are within a safe passage. This configuration is commonly used in situations like tugboats operating in a channel or where there is a defined safe area to navigate.

Navigating inside the lights, beneath them, or outside of them could potentially place a vessel in danger, as these positions may lead outside of the safe navigable area that the lights indicate. Thus, the critical safety measure is to stay between the two vertical red lights to ensure compliance with navigation rules and to maintain a safe mode of passage.