I consider myself a fairly strong supporter of investments in public transportation, yet I often feel a vague sense of discomfort at the way some on the left frame the conversation. A good example is this recent interview that Senator Ben Cardin of Maryland gave to Grist's Kate Sheppard:
I'm a big, big supporter of dramatic change in public transportation. It includes more than just the bus and rail systems in our urban areas. It includes a commuter rail and inner-city rail--the whole gamut of services that get people out of their personal vehicles. I don't want people driving their personal vehicles the way they are today. (emphasis added)
My reaction to this is what I suspect most people's reaction would be: Who the hell is Ben Cardin to tell me he doesn't want me driving? What does it matter to him? Now, look, obviously the problem here is that under current policy automobile travel is subsidized in all kinds of ways, and that eliminating these distortionary subsidies--or, if we're going to keep subsidizing transportation, finding a more neutral balance between cars and other forms of transportation--would cause some unspecified number of people to switch to public transit, or biking, or whatever. So it's literally true that under the course of action I favor, people would not be "driving their personal vehicles the way they are today."
But I think there's a big difference, both rhetorically and philosophically, between saying, "Driving is bad; we want fewer people to drive" and saying "Current policy incentives produce an amount of driving that's greater than the social optimum; we should eliminate these distortionary policies." It's not the government's job to tell you how to travel--it's the government's job to make sure the choices available to you reflect the externalities and social costs associated with them (and carbon emissions are part of the equation here, obviously). But beyond that, government ought to stay out of the way--and I think liberals would have more success if they framed the conversation this way, rather than in terms of "We want you to drive less."