Once you realize the sheer amount of commerce and business that is enabled by a decent metro system you see that they pay for itself several times over.

That's even easier to measure because these gains are localized.

Right. So let's have both! You can't replace the capabilities of roads with metro and you can't replace the convenience of a metro with roads.

Specifically, though, viewing roads or metro as a "loss leader" is probably an inappropriate analysis.

Actually, it has been shown that removing cars increase commerce.

This is a universal outcome with research to back it up?

yes but I am about to go to bed so the link mihht come only tomorrow.

Also some studies have calculated the impact on health in the society and calculated a cost to the society per km of driving a car and the money saved when doing the same km walking or with a bicycle.