Note that in these contexts (comparing with European, Asian cities) tunnels are typically employed to (1) carry mass transit (which requires and in turn support

Note that in these contexts (comparing with European, Asian cities) tunnels are typically employed to (1) carry mass transit (which requires and in turn supports density), and (2) carry through traffic through dense urban centers without having to take up as much surface space. Keep in mind that low-density sprawl is likely to * create much higher infrastructure costs (in terms of streets, utilities, etc) per capita (I believe Strong Towns has written extensively on this topic); * force more people to buy, drive, & maintain a car (and spend more time commuting) when they would not otherwise need to (if you like driving, good for you, but it isn't for everyone).