Which is an acknowledgement that a three lane motorway with a hard shoulder is safer than a four lane road without. So, safety is compromised in the interests of those who want one extra lane of traffic.
Did you mean to write “open” twice in that bit?
I assume you mean eastbound.
That is a good point. However, there are helpful signs at every motorway junction informing drivers of the road layout after the junction. Such as this one at the junction in question:
www.google.com
Yes, but in theory you should do that in the vicinity of the overhead gantry a mile before the lane drop. Doing so beforehand is unnecessary. In practise, anything can happen, especially if the traffic is heavy!
I think laziness/on autopilot is the biggest part of it, given how surprised drivers seem to be when another car appears in the lane in front of them.
100% agree. Often lane 1&2 can be completely empty whilst 3&4 are full.
In certain, limited circumstances, believe that undertaking on motorways is sometimes permitted.
Is undertaking illegal normally? Or is it just against the Highway code, and therefore undertaking would be an example of dangerous driving.
Not strictly illegal, I believe, but strongly discouraged in the Highway Code, except as already mentioned above.
It would be an interesting case though.
Depends on the circumstances. Who's to say that there might not just have been a breakdown in the outside lane during a typical "rush hour", as one possible example.
It happens to me multiple times on a 200 mile journey and there are no incidents.
That sounds like the scenario when a section of motorway is starting to become over capacity vehicles-wise, maybe one tail-gating person applies their brakes, and everyone else behind does so also, and so everything temporarily grinds to a halt. And yes, often happens at relatively low motorway running speeds.
Sometimes you have little choice. If you're in the inside lane, you're not necessarily going to stop (and so not undertake) just because the traffic on a busy motorway has temporarily come to a complete halt in the far outside lane.
Good, this is the point of the rule!
Technically, if you hold a driving license you agree to review the Highway Code regularly (at least once a year?) and keep yourself up to date. I certainly can't claim to have honoured this, and I'm willing to be the overwhelming majority of other drivers haven't done so either.
No it’s because people are terrible drivers and won’t move left. A mile of empty carriageways is all too often. I know phantom jams but should only be classified as such if all carriageways are full
IMO undertaking has to be one of the most dangerous things on a motorway. I rarely drive on them but if going back into lanes 1 or 2 from lane 3 I need my wits about me because of undertaking.
Which means there is every chance you or whoever you were passing were in the wrong lane.
I didn't think all cars came with mirrors ( just like indicators).
I'm old fashioned and I'm always looking in my mirrors and very wary of blind spots. 
Ah yes but what you’ve got to understand is that some cars are so expensive the owners could afford the indicator optionI didn't think all cars came with mirrors ( just like indicators).
Indeed otherwise you have to join the morons sitting in the outside!
I would say "wishing" does not equate to "have to". Nobody (except for an authorised emergency driver) has an abosolute right to create traffic hazards because they haven't left enough enough time for their journey.
