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Major turning points in life are linked with shifts in car ownership and transit use, a new paper argues.

Nobody gets up in the morning and thoroughly considers every available transportation option before deciding how to travel that day. Yet, too often, that's how transportation researchers assume people operate: with an impossible degree of objectivity, rationality, and behavioral isolation. In reality, how we move in and around the city is influenced less by short-term choices and more by long-term events like getting a job, starting a family, or moving homes.

In the past decade some transportation scholars have started to address this limitation with a research approach called "mobility biographies" [PDF]. The basic idea is that daily travel habits form at major moments that can only be identified by considering the full trajectory of a person's life. A zoom lens is great when we want to know how people behave at a particular time and place; a panoramic view explains what motivated that behavior in the first place.

In the July issue of the journal Transportation, a pair of European researchers use this biographical method to determine how, when, and why people in metropolitan Zurich arrive at their preferred travel behaviors. Sigrun Beige of the Institute of Transport Research in Berlin and Kay Axhausen of the Institute for Transport Planning and Systems in Zurich report "strong interdependencies" between key turning points in life and mobility habits.

Beige and Axhausen surveyed roughly 1,200 people of all ages who lived in the municipal Zurich region between 1985 and 2004. The participants responded to questionnaire items on major life events such as marriages, divorces, child-births, residential moves, and changes in employment or education. This same group also indicated any changes over this period with regard to car ownership and public transit passes.

Broadly speaking, Beige and Axhausen found a close connection between mobility decisions and various major life events. After a move or job change, for instance, about a third of all people with partial car availability changed their mobility preference inside of a year — meaning they either bought a car or some type of transit pass. When a move and a job (or school) change occurred at the same time, people changed mobility preference even more quickly. Over time, the data curves for spatial and mobility changes look a lot alike:

The mobility link exists with major personal changes as well. Leaving a parents' house, for example, increased changes in mobility preference. The birth of a child, meanwhile, made a change in car availability more likely.

It's at these critical moments in life that transportation options are "reconsidered and altered," the authors conclude. Once a person hit 30 — the age at which major life shifts begin to stabilize — the probability of a change in mobility really decreased:

In general, persons between the ages of 15 and 35 years are most mobile, i.e., moving and changing occupation as well as varying the ownership of mobility tools most frequently. Afterwards they become relatively established.

The biggest limitation of the work by Beige and Axhausen is that their survey was a retrospective one. Instead of following people over 20 years, they asked people to recall events during this period, which diminishes the accuracy of the response. (To increase reliability, however, the researchers had people report events in six-month blocks.) That's a problem with "mobility biography" in general, though one that should change as the approach develops a longer history.

Still the research reveals some interesting policy points. Since changes in travel behavior tend to occur beside major events in life, proponents of mobility initiatives would be wise to focus on these times. If you want to entice people onto public transit with free passes, it makes sense to do so just after they change jobs or homes, as opposed to at random.

Another important takeaway is that mobility behavior appears to be at its most malleable in youth. That offers another strategic entry point for new transport policies, and it also adds a layer of complexity to the recent trend away from car ownership by younger generations. On one hand, it suggests that young people who don't own cars are likely to remain car-free as they get older; on the other, that may only be the case if they've already experienced many of the major life changes described above.

Charts from Beige, S. and Axhausen, K.W. Interdependencies between turning points in life and long-term mobility decisions. Transportation (2012) 39:857–872. DOI 10.1007/s11116-012-9404-y.

Top image by l i g h t p o e t/Shutterstock.com

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