When is it safe to return to the right-hand lane after passing another vehicle?

The most accurate measure for safely returning to the right-hand lane after passing another vehicle is to ensure that you can see both headlights of the vehicle you passed. This visibility confirms that the vehicle is at a sufficient distance behind you, thereby minimizing the risk of cutting in too soon and causing a potential accident.

Seeing both headlights indicates that the other vehicle is far enough back that it won’t be affected by your lane change. This practice helps ensure adequate space and time to complete the maneuver safely. Other options, while they may have some validity, do not provide the same level of assurance regarding distance and safety. For instance, seeing the entire vehicle in your rearview mirror might not necessarily mean the vehicle is at a safe distance; it could be moving quickly. Signaling your intention is essential in any lane change, but it does not replace the need for visual confirmation of distance. Hearing the other vehicle's horn is also not a reliable indication for timing your lane change, as it could signal various things unrelated to safety. Hence, the requirement to see both headlights offers clear and effective guidance for returning to the right-hand lane after passing.