It's quite a consistent argument when I say that I am in favor of taxing gas and automobiles. In fact, I think they should be taxed at much higher rates, to internalize the costs that are currently being externalized. Ideally, the costs of roads and bridges are covered by those taxes. Thus they're usage-based and not subsidized by those not using them.
I'm not sure why you think the best way to internalize the costs of car usage is to subsidize public transportation. That's just backwards thinking.
As car usage becomes more expensive due to taxation, cities will become more dense. This is the point where we can start talking about building infrastructure for public transportation, that will then make economic sense thanks to population density. It makes no sense to have a bus meander around the suburbs for hours to pick up a few riders, all traveling to different parts of town. It's an impossible idea. You have to have density first. Tax cars and gas so that they pay for their own infrastructure and environmental consequences, and density will be a natural outgrowth, at which point public transportation will be more viable.