Final answer:
To know when you have enough room to return to your driving lane after passing another vehicle, you need to be able to see the front of the passed vehicle in your rearview mirror.
Step-by-step explanation:
When you leave your lane to pass another vehicle, you know you have enough room to return to your driving lane when you can see the front of the passed vehicle in your rearview mirror.
This ensures that you have enough space to safely merge back into your lane without cutting off the vehicle you just passed.