We know that cars cost a lot of money. Aside from the up front capital cost, operations cost people at least $8,000 per year. But ever since cars were produced for the working man by Henry Ford, it looks as if the purchase and ownership of these vehicles has cut into American wealth accumulation. If you look at the savings rate over the last century, it seems to be an inverse relation to auto ownership. This chart created by Scott Bernstein at the Center for Neighborhood Technology shows this relationship. (I've seen this chart in a number of iterations but this is all I could find online)
While everyone thinks that America is a wealthy nation, many people spend a good portion of their money on transportation. But it doesn't have to be this way. Cities that have good transit networks allow people to save money and cut overall emissions.
I want to reiterate that I think cars are an important part of our transportation network, but I think our complete dependence on them in most of the United States will lead many to the poorhouse sooner rather than later. In order to stem the tide, we need good transit. It allows people to pool their resources to pay for transport and allow for the building of wealth.