Originally Posted by
spare_wheel
Shoulder checks have a 180 field of view ... they should not be secondary.
I am not dismissing mirrors. If I commuted in suburban Phoenix I would likely use a mirror. I believe there are situations and riding styles where a mirror can be helpful but I object to the suggestion that a mirror is always essential for everyone and in every context.
As objectionable perhaps as insisting/inferring that a 180° rear field of view is essential before safely changing lanes? Or that no lane change can/should be made without such a view before proceeding?