If i remember rightly, it was a multiple pile-up in dense fog on the M1 and the politicos knee jerked the law.
It is a ridiculous restriction that should be removed. If anyone thinks that 70 mph is a sensible restriction to avoid dense fog fatalities they want their head examining. If they think that 70 mph is a sensible MOTORWAY restriction in good visibility they should be called upon to state why. In other places where there are no practical restrictions pile-ups re not commonplace, and if the 70 mph apologist says that it is a "safe " speed then I should immediately lobby for a 35 mph limit on motorways as that would be even safer.