When should you enter a lane marked with a flashing yellow X?

A lane marked with a flashing yellow X indicates that the lane is only intended for specific use, which is typically for left turns. The flashing yellow X signals to drivers that it is not a general-purpose lane and should not be used for straight-through traffic or any other maneuvers. By entering such a lane only for a left turn, drivers follow the intended use of the lane and ensure that traffic flows safely and effectively.

The other options imply that the lane could be used for purposes that go against the designated markings. Entering the lane at any time or when it is empty would not adhere to the rules indicated by the flashing yellow X, leading to potential confusion and unsafe driving behaviors. Using the lane during heavy traffic, while possibly tempting for a left turn, does not align with the specific traffic control measures in place. Therefore, using the lane strictly for left turns maintains safety and proper traffic conditions.