The Census Bureau has a new publication about commuting in the US as of the 2009 American Community Survey. There isn’t much change from 2000 that’s mentioned, but one table of commute time piqued my interest. This is figure 8, on page 11, showing mean travel time to work by mode of transportation and time of departure to go to work.
It is well-known that commutes on transit take much more time than commutes in a car, but the breakdown based on time of departure defied my expectations. I thought transit trips take the least amount of time at rush hour, when frequencies are the highest, and the most amount of time late at night; car trips should be the opposite. Since people with very long commutes would leave earlier, perhaps around 7 to get to work at 9, the longest commutes by car should be for people leaving to go to work between 6 and 7 or between 7 and 8.
However, it turns out that by all modes, late-night commuters travel the longest. Trip-to-work time declines monotonically with departure time until the 9 am-noon category, where it’s at a minimum (my guess is that people in this category are disproportionately academics and other flex-time professionals who live close to work). This is close to expectation for transit, but for cars, it’s weird: why would people take longer to drive when the roads are clear than when they are congested?
I’d reject an explanation based on leaving very early to work. Insignificantly few people travel 3 hours to work. Instead, the only answer I can think of is that the groups of people who travel to work at rush hour and late at night are of different social classes, and this is reflected in commute times. Late-night commuters are usually low-paid service workers at gas stations, casinos, etc.; those also live on the wrong side of their metro area or in low-income exurbs, and need to drive considerable distances to the favored quarter.
Observe that the long late-night commute trend is the sharpest for carpoolers, who in general skew poor and nonwhite: eyeballing the graph, carpoolers who leave for work between midnight and 5 am travel twice as much time a those who leave between 9 am and noon, compared with a 50% time premium for single drivers (who are wealthier) and transit riders (who tend to work in CBDs, which are equally accessible from all directions).
Obviously, I’m going on partial data here; anyone with access to fuller data could potentially trivially refute my theory. If the theory is true, then two things will be observed in a fuller set of data. First, the late-night time premium will be the longest in large metro areas with favored and unfavored quarters, such as Los Angeles and Washington, and shortest in metro areas with less edge city-favored quarter development, such as New York. And second, if the departure time is broken into more granular categories, then the peak travel time will be at night rather than very early in the morning.