NYC for example has over 6,000 miles of road * a very conservative 60 feet ignoring sidewalks is ~40,000 acres or 20% of the city. Central Park by comparison is only 842 acres. As Property taxes add up to ~55 Billion per year that’s (55/.8*.2) ~= 13.8 billion in lost revenue per year.
There are of course options to build more roads underground, but then they start costing as much as subway systems.
> Roads are cheap and not heavily subsidized when you account for how many people use them.
[citation needed]. You haven't said that fraction of road costs are covered by subsidies (e.g., by property taxes). I suspect it's a substantial fraction.
Your whole point depends on the fact that almost everyone uses roads and of course everyone uses roads when there are no great alternatives. If things were structured differently, the same could probably be said for other transit options.
Bike lanes. for instance, are ridiculously cheap from every perspective, have none of the externality costs you mention, but no one can bike safely as of now and therefore much fewer choose to do so than the infrastructure could support.
Biking is cheap only if your time is worth nothing. It’s also untenable given the climate of most of the US (much wider temperature variation than in Europe where biking is more common), and the physical fitness of your average American.
How much would an equivalent amount of road capacity cost in NYC?
(For the sake of argument, ignore the fact that such an insane amount of road capacity is probably physically impossible in NYC without knocking down half the city)
Even when you add in externalities (climate change, etc.) road subsidies are about $0.10-0.20 per passenger mile. (Of course, electricity on the east coast isn’t generated from renewable sources, so you have to add pollution externalities to the subway too.)
Roads are cheap and not heavily subsidized when you account for how many people use them. There are pollution externalities, but those can be addressed by mandating electric cars without changing the basic structure of everyone having private vehicles.