I like it when someone - even as good as CG - parrots "
Mirror!" at every opportunity
Just me, but I'd much prefer that they didn't tell us there's still a set of three mirrors fixed inside and out of the vehicle, but told us what they
see (or don't) and
why it's important.
OK, just me being pedantic, but perhaps it gets back to the nub of the OP's queston - or, at least, my version answer.
As already pointed out, the key is: 'information', and from that: decisions.
i.e.
Where does most danger come from? Ahead, so that's the way we sit facing in (or on) the vehicle
Where else can danger come from? Behind, so that's why we check to see what implications following traffic might have on our decision making process.
But also? Danger can be created by our own actions e.g. if we try and put the vehicle where someone else is, or soon will be.
So, priority:
1. Course ahead, decisions on that are affected by potential movement of others from behind
2. Course changes to the sides, also affected by the potential movement of others
How to decide which mirrors to use? Timescales & situation
If you're approaching a hazard but won't be altering speed or course, how likely is it that there's something in a constant position alongside in your blind spot? If there's an opportunity for someone to have got there, then you need to check alongside as well as behind.
If, however, you'll be moving your vehicle 'sideways' in the road space ahead, then that door mirror / over shoulder check becomes significantly more important. And if the situation also involves you being, for instance, significantly slower than other traffic could be, then a good check of the interior mirror followed by a door mirror check followed by a shoulder check immediately prior to the change of course could all be needed.
But . . . where does most danger come from? Ahead. So in between all those checks you still need to be aware of the road ahead, and planning beyond the immediate hazard you're planning for.
Isn't that a contradiction?
Yes, so overcome it by building 'mirror history' - don't just 'look at the mirror' or 'look in the mirror', every time there is reduced 'pressure' ahead, take the opportunity to
inform yourself -
actually say out aloud - what's behind, and what it (ie the driver) appears to be doing. Stereotypes and cliches exist for a reason, so use that information too.
I suppose the other key point is to be wary of 'rote' and habit. Use mirror and shoulder checks because they're needed, not because you think you ought.
A classic example is immediately prior to an overtake, when many motorcyclists (I don't know about car drivers) start to close up behind the vehicle ahead, then do a shoulder check before moving out to pass.
i.e. That's a shoulder check having just closed up and halved (or worse) their following distance (and reaction distance/time etc.) . . . Typically they'll be following only a second behind. How long does it take to look over your shoulder and gain any meaningful information? It's a classic (not just for the rear obs.) of a rider doing what they *think* is 'riding correctly'. Usually because they've seen a demo and not fully inderstood when somethings necessary and when it's a mistake.
Long, rambling, stream of consciousness answer - hope it's been helpful
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