Originally Posted by
cubewheels
Also, by turning your head to look to the left or right also helps warn motorists behind you that you're about to change lane.
That is actually one of the great advantages of a mirror is that drivers do not know you are looking back and react to you when you don't want them to.
For example without a mirror you may not realize there is a set of a few cars and a large gap behind them. So you turn looking for a left merge and the lead driver slows to let you in but it is too early to merge left so you have to wave them on and then tuck behind the last vehicle in the set - that slows everyone down and can cause confusion.
With a mirror a quick glance tells you there is a set of vehicles and then you know to just tuck in behind the last one after a final check with head turn and arm signal.
While I can easily look behind me and ride straight I have found a mirror allows for better integration into the flow of traffic than head turns allow for.