The Travel Time Budget
Is our need to travel innate? Last time, I wrote about the intriguing theory of the universal Travel Time Budget (TTB), which states that humans have a built-in travel clock. Perhaps a product of some primeval need to balance exploration and conquest with hanging around the cave and vegging, the universal TTB is said to drive us all to spend about 1.1 hours per day on the go, regardless of nationality, culture, economic system, or era.
In some ways, TTB makes intuitive sense. For example, the automobile has given us the power to cover distances in minutes that would have taken our ancestors hours or even days. Yet we still spend lots of time traveling, making more frequent trips to vastly more distant destinations than our predecessors. Score one for TTB, which says that changes in technology which save us travel time inevitably lead to more travel.
The TTB also seems to have some validity for broad geographic areas. Studies have shown that cities and countries are somewhat bunched when it comes to travel times. A quick check of the American Time Use Survey from 2003-2008 shows that the sample averaged 74 minutes traveling, pretty much right on target with what TTB predicts.
But the closer we look for a universal TTB, the harder it is to see one. Scholars Patricia L. Mokhtarian and Cynthia Chen have reviewed the evidence and find it gets pretty murky.
Take regularity over time. Some studies have suggested that over the past few decades there has been little change in the amount people travel in the U.S. This is what the universal TTB would predict. But other work has found that time spent traveling actually increased. Lavenia Toole-Holt, Steven E. Polzin, and Ram M. Pendyala have found that travel times in the U.S. increased at the rate of 1.9 minutes per person day between 1983 and 2001 (although some of this may be due to improved measurement). So at best, the evidence for a stable TTB over time is mixed.
Moreover, when we switch focus from big aggregate groups to individuals, serious challenges to the TTB concept appear. Individuals actually exhibit a wide variety of daily travel times. The standard deviation in travel time per day for the ATUS population is over 80 minutes, which, given a mean of 74 minutes, indicates a very wide spread in the amount people actually travel.
People differ in travel time based on numerous factors, some of which can be observed and measured. For example, there is general agreement that the young and old travel considerably less than those in the middle of life.
Studies are pretty unanimous about the fact that employed people travel more than those who are out of the labor force, despite the fact that the unemployed would seemingly have more time on their hands for travel. Also, Toole-Holt et al. find that travel rises with education and income.
Travel time also has to do with where you live. A number of studies (if not all) have found that residents of big cities spend the most time traveling; a sister finding is that those living at high population densities travel more (though this finding is nuanced). The association of travel time with urbanization may be due to traffic congestion in dense areas slowing us down, or the fact that those in dense areas have more places to go.
Another factor determining travel times is the duration we spend at our destinations: long stops justify longer trips.
Also, the type of activity we are traveling to helps predict the amount of time we spend getting there. T. Golob and M. McNally have found that we commute about 2.8 minutes per hour of work, that we travel about 7.8 minutes for each hour performing maintenance activities like essential shopping, and that men travel 5.5 minutes, and women 8.5 minutes, for each hour of discretionary travel to things like friends or recreational activities. In short, individual lifestyles do a lot to shape our travel behavior.
Given all of this observed variation at the individual level, how is it possible that we see an overall pattern (about 1.1 hours a day) when everybody gets averaged out? P. B. Goodwin has suggested that there is an interplay in travel times — when one of us chooses to travel more, it causes others to travel less. Perhaps the traffic congestion we each create when we go somewhere dissuades others from making trips, regulating our total travel as a group.
Or there may be some version of travel time budgets, though these are not etched in stone and vary on an individual basis. Mokhtarian and I. Salomon hypothesize that we as individuals do have ideal travel time budgets from which we don’t like to deviate. Yes, we are willing to travel more (or less) depending on need, but the activity of travel gets more or less onerous (or pleasing) depending on how much other travel we do during that day. We might add or drop a trip or two in response.
J. A. Michon proposes that we are creatures of habit who don’t like changes from our daily routine, including our travel routine.
According to this modified theory, when all of our individual travel budgets are toted up, the variations cancel out. So we might have a rough TTB as a nation or a species while still exhibiting widely varying travel behavior as individuals.
So who we are, where we live, and where we’re going all matter, probably much more so than the amount of time it took to bring down a wildebeest or find some wild nuts. Not as neat a picture as the universal TTB, but a much more realistic one.