I used to emphasize and practice that, back when I drove a '93 Honda Civic. It was a useful skill as I would naturally adjust my speed to prevent cars from cruising in my blind spot or to give myself more room for maneuvering.
But newer cars have such poor visibility that it takes too much effort to track movement such that it becomes a diversion. In my '93 Civic any kind of movement in my peripheral--direct or in the mirror--registered immediately; and that peripheral was wide and tall and clear. It was also easier to hear the road and traffic, which also helped. Newer cars make you feel like you're entombed and deaden the senses, compounding the problem.
I know part of my degraded situational awareness is getting older, but since we keep a '98 Civic for when I have to commute by car, I have an idea about which changes are more objective than subjective. The sight lines in the '98 Civic aren't as great as the '93, but still incomparable to the post-2010 cars I've leased or rented. I drive both our family car (new lease every 2-3 years) and the '98 in roughly equal proportion and pretty much all of my unforced traffic errs have been in newer cars. (The most egregious of which are not noticing pedestrians about to cross at a four-way intersection. Modern A pillars seem deviously sized and placed to perfectly obscure the far corners of an intersection. Though I suppose these are the errs I'm most likely to notice as the pedestrians are revealed the moment you begin to enter the intersection from a stop. Most other potential collisions probably go unnoticed.)
Thankfully this year we finally leased a car with lane departure warning. It hasn't actually prevented an unforced err, yet, but I'm glad it's there. (I know it's there because it'll beep if there's an adjacent car when I initiate signaling, even if I'm not moving into the lane. If a car sneaks up after signaling it only warns when you begin the lane change.)