currently doing about 1,000 motorway miles a week in pretty average cars.
worst thing (most dangerous thing?) is where you get a string of cars in lane 3 following too close and drivers keep braking (ie brake lights showing but not much slowing down) this causes a chain reaction of slowing down behind, even though there was no need to.
2nd worst thing = no lane discipline.
IE people pass but will not move back in, I have noticed this seems to be worst in scotland where they all drive along in the outside lane regardless.
I just try to keep to a steady 100 in the hardshoulder thus making steady progress!