When is it permissible to drive in the passing lane?

Driving in the passing lane is permissible primarily when you are overtaking and passing another vehicle. This practice is rooted in ensuring safe and efficient traffic flow. The passing lane is intended for use by vehicles that need to maneuver around slower traffic, which helps to minimize congestion and reduces the potential for accidents that might occur from slower vehicles operating in faster lanes.

In many scenarios, using the passing lane to pass allows drivers to maintain the appropriate speed without being impeded by slower vehicles, which can benefit overall highway efficiency. It's important to note that proper signaling and ensuring it's safe to move into the passing lane are critical components of this maneuver.

The other options, while they may seem reasonable in specific contexts, do not accurately reflect the primary guideline for using the passing lane. For instance, the presence or absence of other cars behind you does not directly justify the use of the passing lane. Additionally, driving in the passing lane at speeds below the speed limit is generally discouraged as it can create hazards for other drivers looking to safely pass. Finally, while a clear road may invite use of the lane for faster driving, it does not define the purpose of the passing lane, which is specifically for overtaking.