When is it illegal to drive in the left lane on a highway?

Answer: A — When you are not passing another vehicle

Driving in the left lane on a highway is primarily intended for passing slower-moving vehicles. It becomes illegal to occupy this lane when you are not overtaking another vehicle, as it can impede traffic flow and create unsafe conditions. The left lane is designated for faster-moving traffic, and using it improperly can lead to congestion and frustration for other drivers who are attempting to pass. The presence of signs prohibiting left lane use, heavy traffic conditions, or driving under the speed limit might apply specific regional regulations or situational factors, but the general rule is that the left lane must be used primarily for the purpose of passing. Thus, not passing while in the left lane constitutes a violation of proper highway driving etiquette and regulations.