I sometimes hear people tell me that they'd love to get rid of their car but they must keep it around for a particular sort of trip. While this is a reasonable complaint on one hand, on the other hand, there is more to switching from cars to public transit than just changing your vehicle from automobile to bus or train. Transit is not intended to be a direct replacement of the private car. The actual alternative to driving is walking. Transit should be considered as a "walking enhancement."
When walking, your personal geography of the city changes. By that I mean, the outlook you have when consulting your mental map of the places you go and the things you need to do around town. Those people thinking about making the switch from private car to walking and public transit sometimes compare their trips under each case. For example, you might open up your web browser to your favorite maps website and compare the Driving directions to the Transit directions. Oftentimes this comparison shows that the transit alternative takes longer. This causes some folks to balk. But it's not the right comparison to make.
For one thing, driving directions never include time to find parking (which can sometimes take longer than the trip!), and they only occasionally factor in traffic delay. But I'm talking about something deeper: the choices you make traversing the city on foot are different than the ones you make while driving.
Some choices are obvious: you look at options that are closer to home; you frequent local business; you don't expect to carry too many things. You don't make trips out to the giant mall complex by the highway unless you really must. You keep an eye on what's available near stations, bus hubs, or along frequent transit lines. You look for clusters of "microdestinations" where you can quickly walk between the places you need to go. Distance and time projections are malleable: places that are more difficult to access seem further away, while those easier to access seem closer. The quirks of your local transit agency play into this: mistakes made in network design inflate travel time, while well-functioning connections decrease it. (This applies to road networks as well).
Not everyone can handle this change of viewpoint, and there's nothing wrong with that. But for those who are considering the change, it's not sensible to try and compare trips directly. You're not going to follow the same patterns of travel on foot as you would with a car. You're not going to run out to the faraway mall for small purchases, and attempting to duplicate that experience using the bus will likely be highly dissatisfying. You will factor transit accessibility into your decision about what jobs you take and where you live. For example, I have heard one person tell me that they chose one job over another because the other would require the purchase of a second car, which is very expensive. I know that lots of people choose to live near the T because it opens up those options for them.
This observation works both ways: buying a car means that you are going to shift from frequenting local walking-oriented places to those sprawled out on the highway, where you don't have to contend with limited parking. You'll choose to live and work somewhere with easy highway accessibility if that's what's important to you. It's not possible to separate decisions about lifestyle from land use and transportation. They are inextricably tied together, regardless of mode.
I believe that a lot of woes of city planning over the past century can be traced to the understandable desire to have it both ways: catering to vehicles and walking equally. But it's not really possible, and the result can be the worst of both worlds: sprawl with dangerous roads, yet a constant "shortage" of parking. Dis-investment in communities, followed by misguided "urban renewal" and the destruction of cities.
Everything I've said is a corollary of city geometry: shorter distances are more amenable to people on foot, but provide fewer places to conduct and store bulky vehicles; while large distances provide ample space for those vehicles, but are an obstacle for walking. This all may seem obvious, but for some reason, I find that it often gets overlooked.