Depends what you use tunnels for. Subway systems encourage density in cities. In a plane, you can see the Toronto subway lines based on the density around the s

Depends what you use tunnels for. Subway systems encourage density in cities. In a plane, you can see the Toronto subway lines based on the density around the stations. But new subways tunnels are the big expense that blocks more of then being built. (Though I've heard station construction also is a big cost). Dense areas lead to more walkers, less drivers, cheaper overall infrastructure. It's not about having more infrastructure to maintain, it's about replacing the expensive stuff we have with efficient infra instead.