We usually don’t think about this actively, but generally the way people decide when to leave in order to get somewhere on time is (Desired Arrival Time)-(Transit Time). Easy enough, if I need to show up at the dentist at 8 and it takes 15 minutes to drive there, I leave at 7:45.
However, most people do an awfully careless job of estimating “Transit Time.” As any statistician knows, the expected value of anything is the sum of (relative frequency)*(possible outcomes). In this case, possible outcomes (in time, since we want the answer to be in time as well) include anything that could go wrong on the trip, and obviously the relative frequency is the likelihood that it will go wrong on any given trip. So a good way to estimate transit time would be:
Transit Time=Normal Error-Free Trip*Probability of that happening+Flat Tire*Probability of that happening+…
And on and on. All of the “probabilities of that happening” should add up to 1, with the probability of a normal trip occurring being something large like .8 and all the other ones combined making up the rest. All of the values for things like “Flat Tire” should be larger than “Normal Trip,” because the time value for that event is normal time plus the delay.
Now for an example with made up numbers (I have no idea how likely any of these catastrophes are). The example is assuming this is a 15-minute trip, unadjusted (pure driving time).
Expected Value of Transit Time=(Normal Trip)(freq. Normal Trip)+(Flat Tire)(freq. Flat Tire)+(Adverse Weather Conditions)(freq. Adverse Conditions)+(Accident, other cars)(freq. Accident, Others)+(Accident, participant)(freq. Accident)+(Police Encounter)(freq. Police Encounter)+(Mechanical Trouble)(freq. Mech. Trouble)+(Alien Invasion)(freq. Alien Invasion)+(Construction)(freq. construction)+(Wrong Turn)(freq. Wrong Turn)
500: Depending on the severity of the invasion
This is about 23% longer than the original, wrong estimate of 15 minutes. On a long trip, this 23% could be a big deal. Hopefully by reading this each of you will be more conscientious in your estimates of driving time and I will have done my part to improve the punctuality of the world.
Unfortunately, always leaving 23% early will get you to your destination 23% early 80% of the time, and exactly on time almost never—most of the delays in the equation are of longer than 23% (so you’ll still probably be late). You could argue that the 23% of time you save on each uninterrupted trip is worth it even though you’ll be later in that 20% when anything bad happens. So as these things always are, this is open to interpretation. Get it together though, world, be on time.
There should actually be a scalar to multiply the time by based on how bad it would be if you were late for whatever the thing is, but I’ll let you figure that out.