Wasting fuel at a rate of knots or gaining time at a rate of knots ? One could apply the same argument to any lower speed so it's not really applicable. Speed should be regulated by safety levels, and motorways are the safest roads aren't they ? In decent weather the fact that most go more than 70mph and the road is still the safest around suggests that the limit is too low. The major issues are tailgaters, slow vehicles in the right hand lanes, those who constantly chop and change lanes, under-takers, those who think putting an indicator on is an automatic right to pull out, those who come out in your braking space instead of looking in the mirror for a reasonable gap, those who cut you up pulling back in, those for whom look/signal/manoeuvre are tick boxes not stages and so indicate as you are about to pass thus worrying you that they're daft enough to intend to pull out in front of you. Probably covers most issues. Oh, and cars should have blind spot mirrors or cctv or something; it's terrible folk can actually look and not see before making a move.