Yes, the equation is simple: those who live downtown in an urban environment take transit frequently. Downtown, a streetcar or subway is usually less hassle than driving. In the suburbs, the proportions are reversed and everyone drives because taking transit there is a nightmare. When travelling between the suburbs and the downtown, the transport method will probably depend on the specific route.
Basically, most people will take the path of least resistance. It's not reasonable to expect that people will just "do the right thing" and take transit if they have a car sitting at home that can get them where they want to be faster.
That's why the solution is not to goad people into changing their habits; the solution is to change our transit and road systems so that taking transit becomes the logical easier choice for most people. How to do that is the challenge.
My family and friends in the suburbs never take transit, and they think I'm a bit loony for living without a car. My friends downtown use transit almost exclusively -- only a few own cars, and those that do use them mainly for visiting the suburbs.
