I also get annoyed with the everyday late people, but don't start a MN post on them!
But I did read something interesting about some personality types, basically they don't mentally plan for the time it takes to get ready & get there.
So if they are due somewhere at 9 and it takes 30min to get there and 30min to get ready, at 8am most people are saying "time to go, let's get ready" kind of thing. Things can slip, or some might start this at 7:45 to have that extra 15min up their sleeve.
But some personality types don't think like this. At 8am they still think they have stacks of time, at 8:30 they don't consider themselves late, as they still have 30min (even though it takes an hour to get ready etc)... so they still idle about, rather than screaming like a banshee and getting everyone out the door in 5 min. Finally at ten to 9 it becomes apparent to even them that they are hiddeously late, and they rush about and try to get there in 10mins (assuming they care about being on time, even if it is now impossible). They moan and whinge that everyone didn't get ready, traffic was bad, someone needed something as they walked out the door - but really it was the lack of time beforehand, not the other excuses.
It seems so bleedingly obvious to me that if it takes an hour to be ready and get there, that if you start this anytime after 8am you are already running late. And with something like the school run, it isn't hard to work out the typical getting ready and out the door times - you do it hundreds of times a year.
But apparently some brains just don't/can't factor in that time of preparation/travel.
Still very annoying, especially to the teachers.