When I fly, I try to schedule a departure at the airport as early as possible. Planes leaving at six or seven in the morning tend to be less crowded, and leave on time, because the delays and problems haven’t had time to stack up, and the kind of people who schedule early flights are also the kind of people who get there on time, so on a lucky day you can even leave early.
The L train at rush hour displays similar behavior, which I had forgotten until this morning. For no apparent reason, I was wide awake at 6:30. After making coffee and watching The Daily Show for a bit, I headed to the train with every intention of making it to work by 8:45. I had two things going for me: I was leaving an hour earlier than usual, and I can combine the hat I recently bought with my hoodie and my shades to do a passable impression of popular image of a gang thug, so people get out of my way more quickly.
The Departure Paradox states that if you commute at the particular point on a subway line where the trains can no longer absorb the rush hour traffic, it does not matter what time you leave during the hour of the rush. In my particular case, the rush hour is between 8:15 and 9:15, and if I exit my apartment door during this time, I will arrive at a random time between 9:30 and 9:45 regardless of when I leave.
Once the traffic volume reaches critical density, problems start piling up. A train is no good to you if you can’t get on it, but frustrated commuters will try anyway, thus delaying the trains, thus creating more trains with too many people, etc. As the trains get fuller, probability demands that someone will have a medical emergency as often as not and pass out at a major station. Delays multiply and feedback and a twenty-minute train ride rapidly, so to speak, becomes an hour long. This process is already in full swing by 8:30, and slowly winds down, ending by 9:30, so the later during the rush hour you leave, the shorter your trip is likely to be.
Today, the L train made it to the river, then turned back due to a sick passenger three stations into Manhattan. I gave up and worked from home, on the grounds that if a 400 ton train couldn’t make it across the river, I wasn’t going to either.