But there's a evolutionary problem here. You don't have eyes in the front and back of your head. Bikes are small and hard to miss. They can easily slip into your blind spot while you are looking at the road and switching to look in the mirror.
You're right, a driver is definitely required to monitor what's coming from behind them when making a turn. But when we consider limits in eyesight, moving into the lane keeps everyone safer.
You're right, a driver is definitely required to monitor what's coming from behind them when making a turn. But when we consider limits in eyesight, moving into the lane keeps everyone safer.