Surely in fast moving traffic on a motorway you should be aware of
everything that is going on around you :shrug:
Which means checking mirrors and looking ahead all the time so you know where other vehicles are if you need to change lanes etc.
Undertaking seems to be a fact of life these days, possibly caused by lane hoggers, (I've had impatient drivers pull into the inside lane to pass because they won't wait for the outside lane to clear, which is what I am doing), if I moved into that lane, say to exit motorway, without being aware of what was happening who's fault would it be if I hit them as they were in my blind spot in lane that had been clear for ages :shrug: