I cant take a perfect judgment by looking the mirros while lane change. Please tell me how can I understand the safe distance of behind vehicle in mirrors ?
If you want to know if you have enough room to move in front of a car, view that car in your rear-view mirror. If you can see the headlights, and the car isn't moving faster than you are (coming closer), then you have enough room. Of course, you need to make sure that you check all of your blind spots. But if they're clear, the car's not moving faster than you are, and you can see both headlights, then you have enough room.