How about being able to build taller buildings in order to support higher population densities in major cities?
Most cities in the US have all kinds of zoning regulations that prevent cities from reaching higher population densities: https://news.ycombinator.com/item?id=11918230. For example, there are quite a few livable cities in the world supporting > 15,000 per sq km, most cities in the US hardly reach half that amount.
This is definitely not a free market phenomenon; I am pretty sure that in a free market (or even a market with rationally determined, not NIMBY based regulations), there would be increases in population density of major cities in the US.