Obinna, I was thinking about this. If you moved the max_time and min_time functionality into the method in WalkingData, doesn't that break the use of this as a standalone function? There are still `max_time` and `min_time` arguments in this function but they don't do anything.

Maybe the best thing is to make a separate function that deals with the max and min time stuff, so that you can use it in both the grf functions.