Final answer:
It's safe to move back in front of a passed vehicle when its headlights are visible in your rearview mirror.
Step-by-step explanation:
When passing another vehicle on the highway, it's safe to move back in front of the vehicle you have passed when you can see the passed vehicle's headlights in your rearview mirror. This is a general rule of thumb indicating that you have enough space to re-enter the lane without causing the passed driver to slow down or take evasive action.
It's important to take into account the speed limit and to ensure you're providing adequate space for safety. In towns and cities, where the speed limit is usually 60 km/hr, you can gauge this by considering car lengths; you can safely move in once you are several car lengths ahead.
This safe distance allows both you and the other driver enough time to react to any potential hazards that might arise suddenly on the highway.