Josh Barro calculates the public and private money the US sinks into automobiles and their infrastructure:
All told … $1.08 trillion was spent on road travel, with government subsidies providing only $83 billion of the total. That’s a subsidy of less than 8 percent. Even if you add the implicit subsidy from excluding gasoline from general sales taxes, you push the percentage to just over 10 percent. No transit system in North America operates on a subsidy that small. So while the transit advocates’ general attitude is right—any subsidy for road travel is too much, and drivers should pay their own way—they overstate the importance of those subsidies in shaping how people get around. Incentives matter, and if you ended subsidies for roads, people would drive less—but probably not much less.
He goes on to argue that "the real culprit keeping Americans away from mass transit and inside cars isn’t subsidies; it’s planning and zoning."
The Dish 