What should you do if you can no longer see a vehicle's headlights while passing?

When you can no longer see a vehicle's headlights while passing, the correct action is to return to your lane. This is important for maintaining safe driving practices. Losing sight of the headlights indicates that the vehicle you are passing might be getting closer to you, and returning to your lane ensures that you avoid a potential collision as you might be in the way of the approaching vehicle.

Returning to your lane is a defensive driving maneuver that enhances safety. It's essential to ensure that there is enough space to merge back without interfering with the oncoming traffic. This choice reflects the need for caution and awareness of other vehicles on the road.

In contrast, speeding up to get further ahead may not provide enough time to safely merge back. Slowing down and signaling could potentially confuse the driver behind you, especially if they assume you are going to continue passing. Remaining in your lane would not be safe either, as it risks a collision with the vehicle you are trying to pass. Thus, returning to your lane is the safest and most appropriate response in this scenario.