I used to drive trucks and in my time came across allsorts police pulling you for going slow as they thought you were tired which meant then you were flat to the mat to make up for a half hour check and your boss asking what you did to get pulled was one of them as I'm sure you will have come across in your bus driving. Fair play 50 when I was driving was not that slow as motorways were still quick and you were passed by everything, but now with the amount of traffic 50 in the inside lane causes congestion as some trucks are limited to 53 so coming upon someone doing 45 to 50 in traffic is a big issue also they leave passing you very late cos they cant get out to pass even when they are indicating drivers still stick with the middle lane.
if I'm traveling with my caravan I go in the middle of the night I can sit at 50 and reach my destination before the traffic builds up. I suppose the main point I'm trying to make is on 3 or 4 lanes if someone limited to 53 comes to someone sitting at 50 or less it screws them unless the users of the middle lane pull over to the outside lane which for some god forsaken reason they will not do, so better motorway driving skills need to be practised as iv met people who believe that lane one is 50 mph lane 2 is 60 mph lane 3 is 70 mph apologies for my speech.