4 aspect, compromise of principles?

[PJW]

Hi,
I have recently read :
"4 aspect signalling, reduced overlaps and permissive working are all examples of signalling principles made flexible to meet operational needs."

does anybody know how exactly 4 aspect signalling falls into this category?

thanks
fil

I guess in the historic context that the warning to brake to a stop signal had always been given by one signal at braking distance. Hence it must have been a bit radical to have proposed a 4 aspect system whereby a waring to brake is given at a signal to stop not at the next but the signal beyond that and to have "overlapping" braking distances for alternate signals. With the perspective of hindsight, we just accept the philosophy but it probbaly didn't seem like that at the time of introduction.