"Something always comes up"

This is the most sincere objection, and it's the one I'd most like to take apart. You leave in time. Then the phone rings, or the child can't find a shoe, or the printer jams, or there's an accident on the ring road. It isn't that you cut it fine. It's that life intervenes.

I believe you about each of them. Now look at the list. The phone, the shoe, the printer, the traffic. Did any of them come up for the first time this month? Or have they come up, in one form or another, most weeks for years? The individual event is unpredictable. The fact that there'll be one isn't. It's the most reliable thing about your mornings.

Justin Kruger and Matt Evans published a study in 2004 whose title says the finding. "If you don't want to be late, enumerate." Unpacking a task, which means listing the separate things it involves before estimating it, reduces the planning fallacy. I couldn't get hold of the paper's summary, and the notes say so, but the point is one that follows from Buehler's work. When you estimate a trip as a whole, you picture it going well and produce a small number. When you list its parts, the parts bring their own small delays into view, and the number goes up towards the truth. "Leave for the meeting" is one item. "Find keys, find notebook, coat, lock door, stairs, park, wet path if raining, stairs, corridor" is nine, and each of them has taken time before.

There's a general idea behind this, and Daniel Kahneman and Amos Tversky set it out in 1979. You can look at a task from the inside, imagining how it will go, or from the outside, asking how things like it usually go. The inside view gives you fifteen minutes. The outside view asks how long your trips to that building have taken over the past year, and answers twenty-two. The outside view isn't interested in why last Tuesday was slow. It only counts.

So "something always comes up" is an accurate report, and it's the strongest argument there is for a cushion. If you know there'll be something, you don't need to know what it is. You add ten minutes for it. On the mornings when nothing comes up, you're ten minutes early, which chapter 6 argued is a small price and chapter 18 will argue is barely a price at all.

One caution. The cushion needs to be sized to the ordinary bad day, not the extraordinary one. A motorway closure or a child being sick isn't what the cushion is for. Those are the rare occasions on which you'll be late and can say so without embarrassment, because you'd done everything reasonable. Chapter 20 covers them. The cushion is for keys and shoes and the wet path, and those you can count on.