Mode choice versus life cycle change

During TRB I attended a presentation on the effect of life cycle changes on travel pattern characteristics. The presenter defined the usual life cycle changes (getting married, changing home location, having a child, etc) and set up a structural equations model to related these changes with the size of a person’s social network, the length (distance) and number of trips per day, the length (duration) and number of activities per day, and so on.

The work was interesting and got me thinking whether one could treat “being green” as a life cycle choice rather than as a mode choice. In the usual mode choice context, one considers each trip and weighs the costs and benefits of each mode. In theory, each person will choose the mode that is best for her or him. It is assumed that people are perfectly rational, and any differences in choices given apparently identical attribute sets is assigned to some unobservable error. While this ignores the fact that people are pretty bad at making quick and accurate appraisals of the true costs and benefits of different modes, one could argue that even this fact is taken into account in that different people have different tolerance for spending time and effort searching for a better solution. In the end, the random differences between people are presumed to come from some known distribution, and the modeling exercise estimates the relative weights of the observables (cost of bus, cost of transit, cost of biking, cost of walking, etc). In the end, the analyst has a model that can estimate the answers to questions like how many former drivers would bike if bike travel times (or generalized cost) were decreased 1%.

Applying the idea of mode choice to my trip home today, in the mode choice modeling world my choices would consist of walking, biking, bus, and car, and most models would show that car is the hands-down winner in terms of high benefits and low costs. This is in fact true for me too—I own a car, and I’m already paying taxes and insurance on it, and it is a fast and easy way to get around. The fact that I am going to choose to bicycle home this evening would be attributed to some unobserved randomness in how I personally evaluate those costs and benefits.

This isn’t what happens at all. In reality, I have a beautiful Atlantis Rivendell sitting next to my desk in my office, and I am going to ride it home. I chose a “bicycling lifestyle” quite a while ago, perhaps not even consciously, and so I generally choose bike first unless there is some major constraint which requires me to use a car. Those constraints usually involve my wife and my children, or more accurately, when my lovely wife is out of town and I have run my children around to their after school activities. Which brings me to my real point of this post: bicycle choice is a life cycle stage for me, not a mode choice. This stage or state is not one that my wife and children are in, and so whenever I have to participate in their daily activity patterns (ballet, piano, theater, etc), I am forced out of my bicycle life cycle state, and into a car life cycle state.

At the same TRB conference, I was looking for research in the domain of active transportation, or how and why people choose more active travel modes such as bicycling and walking. This was inspired by my wife, who has a master’s degree in public health and told me that there was a growing interest in trying to use active transportation to combat serious health concerns like obesity and diabetes. In a nutshell, people drive too much, and sit in front of the television too much, and modern humans are becoming fat slugs. We didn’t evolve to be slugs, and so our bodies are not dealing well with this new set of circumstances. One way to end the cycle is to “get” people to walk or bike more when they travel.

The trick is that the car is just so great at providing a low cost, high benefit transportation solution. It is nearly impossible to identify a normal trip example in which driving loses out to walking or biking. Cars have great roads and priority access at intersections and in parking lots, have super-fast top speeds, and plenty of hauling power. So how can you expect someone who is overweight and diabetic to choose to walk to the supermarket (most likely across major roads and through a massive parking lot) to buy groceries?

In some places the “active life style” is most definitely imposed by the urban form, and driving to the market to do some shopping makes no sense at all. For example, when I lived in Boston, the markets were tiny, and even if I did own a car I certainly wouldn’t have given up a parking spot just to spend 20 minutes looking for another one. Instead, I shopped as I walked home each night as I passed different stores. If I needed vegetables, I’d swing through Beacon Hill. If I needed meats or cheeses, I’d go a block out of my way in the North End to the deli I liked. Apparently, there are cities all over the world who are taking steps to try to make their urban environments more walkable, which at the same time will make them at least slightly less drivable. I interpret this movement as trying to “get” people to be more active by trying to improve the benefits to costs ratio of active travel modes versus driving. But the fact is you can’t make every city into Boston, and just a few short miles from where I lived in the North End, driving was the norm, not walking.

After that TRB session, I think there might be a different way to approach persuading people to choose active travel modes. I think it is important to view active travel as a life cycle change. Choosing to walk or bike is going to have a drastic impact on every aspect of a person’s usual travel pattern, not just on a single trip. This means that one who currently drives cannot simply pick any random trip in any random day and choose to perform that trip by bike or on foot. Rather, the person will need to rearrange many activities in order to allow for the time and inconvenience of the non-motorized mode. These secondary impacts are not trivial, and work against the active transportation choices. Perhaps by viewing the choice of an active travel mode as a life cycle stage and by applying some of the same research tools we’ve applied to how more traditional life cycle changes affect travel patterns, we can devise ways to help people to shift to active transportation and lead healthier lives.

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