Looking at images of Paris and New York's Lower East Side, I realize that very dense, older urban areas are not particularly high rise. In many cases, they fail to exceed 4-5 stories. But I would estimate a Floor Area Ratio of something like 400%, which generates transit suitable densities. What is critical in generating sufficient density for transit appears not to be tall buildings, but the lack of parking. Every car takes up 300 square feet per parking stall. Include landscaping and access lanes, that means 10 acres of land only provides about 900 parking spaces. Keeping that same ratio, a 1 acre parcel could be used to produce 90 surface parking lots. Or (at 350 ft^2) about 125 small offices. But if each office is matched to a parking stall, so the net production is about 62 offices.
Structure parking is enormously expensive, because it has to support a 'live load' of cars, rather than people. Thus, as much as possible of the need for parking will be met using surface lots. Assuming a 1:1 office:parking ratio, 2 stories of office mean that 2/3 of the parcel must be devoted to surface parking. With a four story building, 80% of the lot will need to be devoted to parking. Automotive oriented high density will produce a city of vast towers, islands in a sea of parking--hardly urban.
Thus, generating urbanism is not just a matter of generating density--it is a matter of providing density without providing parking. Which means alternative modes of access.