be capable But are most modern drivers? Given a car is only as responsive as the reaction time of the person behind the wheel, and given the number of people that trundle around well below the speed limit, pulling out without indicators, or using their mirrors, I would suggect 100mph on motorways as a general rule would not be safe, especially given the much larger differential in speed it would create between cars and lorries (which often fully loaded and up a hill arn't capable of much more than the 55mph maximum anyway)" "I was going to just agree that the limit should be 80 but then after reading some comments thought about other factors. Most cars are capable as going 80mph relatively efficiently but if I had 1.0 car 80 is quite high. If I had a turbo injected petrol engine I wouldn't always want to go 80 as my fuel was be burning a massive hole in my pocket. My issue would be there would be too many different speeds being driven on the motorways. Lorries struggling uphills, like Antony_Rol said, the smaller cars struggling at 80 and everyone else driving at the speed or over. I know there's a lot now but increasing the range will only make it worse.