Recently, I've argued that we would expect an S-shaped shift from private cars to transit if the following conditions hold:
Efficiency: for every dollar of subsidies, transit gives people more access to places where they want to go than cars do.
Demand: people patronize systems depending on the access that they offer.
Representation: subsidies are distributed based on demand.
I've now discussed efficiency and representation. I've touched on demand before, but it's the kind of thing you can say a lot about: what factors go into a person's mode choice? Recently, EngineerScotty and Jarrett Walker have also discussed some of the motives for mode choice. But before I do that, I think it's important to observe that "mode choice" actually covers several different choices that people can make.
You might think that a mode choice was a universal, permanent decision, such as "I will walk everywhere I go," or "I will always take transit." But that actually doesn't happen, unless someone's really trying to make a point. Even the most diehard walker will probably take a ferry if there's no bridge available.
At the opposite end of the spectrum, but pointing in a similar direction, no transportation system provides door-to-door service for every possible trip. It may not matter for our goals that someone walked the thirty feet from their front door to the car, but it may matter that once they got out of the car, they walked a full city block length to get out of the parking lot.
One kind of mode choice is per trip, based on particular circumstances ("It's icy, so I'll leave the bike at home") or as a trial. Often, it's based on whether the vehicle will be an asset or a burden later: someone may drive because they're planning to go straight to their country house after work, or alternatively they'll take transit because they're planning to go out drinking after work. In some circumstances you can even decide on the fly for a particular leg of the trip ("I didn't see a bus, so I hailed a taxi").
If someone makes a long-term mode decision, they often don't make it overall, but for a specific class of trips. One common scenario is the suburban resident who gets to work by bus-walk or walk-train-walk, but drives to the supermarket and the movie theater. Some people have fantastically complex arrangements that they can maintain for years: subway to work, commuter rail to visit the Stamford office, car service home from work, taxi to go to the doctor across town, car to go to Vermont for the weekend, bus to get to the museum, subway to meet a friend in Greenwich Village, walk to the deli, rollerblade to the park, fly to Punta Cana.
There are also long-term commitments and investments, the most obvious one being buying a car, a bicycle, a car-sharing or bike-sharing membership, or a transit pass. Other choices include the location of your residence, how much off-street parking it has, your workplace, whether you have to travel for work and how much and where, your favorite social hangout, your shopping, your child's school, a weekend or summer house, place of worship, and restaurants. Any of these locations may be better or worse for transit, driving, walking, bicycling or skating. They will all affect long and short-term mode choices.
One of the most frustrating things for me during the congestion pricing debate was to read comments like, "It's not a choice! I need a car! I need to drive! I need a free parking permit! How else am I supposed to get from my home in Rockland to my kid's school in Manhattan and my job in Brooklyn?" What was even worse was when instead of "I" they said, "some people, unlike you elitists..."
Since our proximate goal is to get people to stop using cars, we care about all these decisions: the short term and the long term, and the commitments that influence them. The good news is that there are tons of ways that the environment can be changed to encourage people to choose transit, walking or cycling more often. The bad news is that these can be changed in the other direction too.