In which situation is it permissible to cross the middle white line?

Crossing the middle white line is permissible when the line is dashed. This dashed line indicates that it is safe for drivers to change lanes, as it signifies that passing is allowed. The dashed line serves as a visual cue for drivers, allowing them to execute maneuvers such as overtaking another vehicle or changing lanes when it is safe to do so.

The other scenarios mentioned are not appropriate for crossing a continuous white line, which typically suggests that lane changes or passing are not permitted. Heavy traffic does not change the rules regarding lane markings, and while overtaking might seem like a reason to cross the line, it specifically applies only when it is dashed. Emergency situations can complicate driving but do not justify disregarding lane markings unless absolutely necessary for safety.