The idea is to be sufficiently aware of your surroundings that you know somebody is in your blind spot because you saw them go into it, and they haven’t reappeared yet. You should be checking your mirrors pretty much constantly while driving at speed, every 5-7 seconds is what I was taught, and what I stick to. To those who say that you can’t take your eyes off the road in front, you are either driving too fast for the conditions, tailgating, or taking too long to process what is going on in the mirrors, probably as a result of under use, you are not accustomed to the view and are having to decipher what is going on, rather than being able to tell at a glance, as you would if you were checking them as often as is prudent.
Two quick looks are better than one long one, and you should know just as much about the cars around and behind you as the ones in front of you.
You need to constantly ask yourself “What if?”, what if the car in front brakes suddenly, can I stop or would I have to swerve? If I can’t stop, which direction (if any) is safe to go? What if another vehicle signals to move into my lane, can I slow down to let them out, or are other drivers tailgating me? What if there is a queue over the brow of this hill/round this bend, have I left room to stop safely?
Most of the time, all is fine and traffic keeps moving, which can make you less prepared when something does go wrong, and you have to think in a split second before acting. If you do the thinking all the time, it becomes routine, and you then have more time to react in an unusual situation, helping to prevent accidents.
I’ve lost count, as I’m sure many of you have, of the amount of times I’ve seen a line of tailgating cars have to slam on their brakes in the overtaking lane, because somebody pulled out without looking, or in front of someone who wasn’t looking. With enough tailgating cars, it becomes a pile up, and can block motorways for hours whilst it is cleared up, and it’s completely unnecessary.