You're saying the same thing that I'm saying.

Subsidized roads creates an arbitrage that allows people to live away from their jobs. This concentrates commercial activity, i.e. it creates commercial-only zones and residential-only zones that are far away from each other. Prior to that, people lived and worked in the same area. These were very dense areas, but the economic activity of the region wouldn't be described as concentrated because there was no gradient to low-activity surroundings.

Obviously actual zoning policies play a (significant) role here as well.