Under what condition can you legally drive in the left lane on a highway?

Driving in the left lane on a highway is primarily intended for passing slower vehicles. When you are passing another vehicle, utilizing the left lane is not only legal; it is also in accordance with safe driving practices. The left lane is designated for overtaking and should be used for that purpose to facilitate smoother traffic flow and to avoid congestion.

In contrast, driving in the left lane simply because it is the fastest lane, or because you are in a rush or feel like it, does not adhere to the rules of the road and can lead to unsafe situations, such as causing delays for other drivers who are attempting to pass. It's important to use the left lane appropriately to promote safety and efficiency on the highway.