US and Japan both have a number of mid-tier cities in the 4000-6000 per km2 range (look at the "list of [US|Japan] cities" pages on Wiki and sort by descending pop density). Japan has a number of efficient cities far from the Tokyo/Osaka megalopoli that don't benefit from their network effects. Consider Fukuoka, Sapporo, or Kagoshima (all fairly remote/isolated cities) compared to Miami, FL and Santa Ana, CA (for high-density US cities outside of the Northeast Corridor). Hiroshima and Sapporo have surprisingly-low pop densities closer to Nashville and Kansas City. We Americans should be able to draw some applicable conclusions even when we look outside of Tokyo. The initial reaction is usually "the density disparity makes it cost-ineffective when applied to the US". If we zoned and developed along Japanese patterns, wouldn't our city densities increase, due to the higher quality of life delivered by the efficiency improvements? People would actually want to live in places where they had flexible transit options and safe walkable neighborhoods with integrated commercial and entertainment activities.