The maths might work like that but reality doesn't as you get faster. The guy who thinks he's nominally doing 90mph should be shaving a third off his journey time compared to 60 but in reality he's just rushing to the next congested area, when the slower traffic will catch up and it all starts again, like a safety car in a race. That's why older sat navs without live traffic updating still gave good estimates. Bike magazine took a cruiser and a hot sportsbike and told the riders to have a fun afternoon however they liked, average speed for both was 63mph, even though the sportsbike rider thought he was 'on one'. However fast you think you're going on the motorway, I guarantee your average speed will be mid sixties at most (unless its 4am).