A sudden patch of really thick fog could be all that it took to bring about this tragedy.
Imagine plunging into a pocket with virtually zero visibility. What do you do? Plough on regardless hoping for an improvement?
Brake instinctively? In which case someone who like you suddenly can't see, shunts you or swerves into the path of someone else?
The dilemma is probably faced many times over the country and nothing horrible happens, but in these conditions it only takes one collision then "safe following speeds and spacing" are not enough.
I don’t have any figures to back my claim but I believe that a large number of motorway crashes happen at junctions, From what I have seen when driving on motorways a lot of people don’t understand how to use a slip road either to slow down or to join the traffic at a similar speed. Fri morning on the M25 (J9 – J11) was a accident waiting to happen the speed some people where driving in heavy rain was too fast, the guy in the 2nd lane doing 40mph being passed both sides by cars and trucks was just as bad. The AA rep on BBC news pointed out people drive to close to each other and tailgating should be a prosecutable offence, I totally agree.
"vehicles that won't move over or slow slightly to let you in off the slip road"
I am not sure how a driver can move over slow slightly? In fact this seems to be an assumption by many drivers and has to be a significant cause of motorway accidents. Vehicles joining a motorway should match the speed of the motorway traffic and merge with the flow.