Riding on the road one needs a good helmet or glasses frame mount mirror. It may take a week to get used to it.
Do not set it in your line of vision. It needs to be off to the side so you need to turn your head to see back. Otherwise it gets in the way. Bar mirrors only work when your head is in one place, with road bars a different grip moves your head to a different place where the bar mount mirror won't work sometimes.
The motorcyclists have two good mirrors, and they don't get passed all the time like a bicycle. They won't be surprised by something from behind. With a good mirror one can see vehicles coming from a long way off. As the vehicle approaches move left and see if the car moves left to get by you. Then when the vehicle gets close move to the right into the space you made for yourself. With enough practice this gets pretty easy. I can actually see forward and backwards at the same time for just a second or two. Then I need to do one or the other.

I'm amazed people don't use mirrors on the road especially in traffic. Not having one makes no sense at all. Yes I can turn around and look back without making the bike weave. But it is much better to not have to. I stopped being startled from something behind me ten years ago, when I got a helmet mirror. The first time I used one, I gave it back,it caused eye strain. Then when I realized the first rider in my group could always see cars before I could, I tried it again.
It did take me a few days to get use to it, and I was told to have it off to the side more.