People drive through long tunnels or ride on subways every day. Being underground is not a problem.
Subways are categorically a different animal. Of lesser endurance than light rail networks, the excess of amount of time spent within them is due to unintended delays and speed limits that draw out travel times. Spending more than 45 minutes on a subway trip is often viewed as unhappily inconvenient and/or an error.
Aside from unusual traffic delays, you are probably right about the 30 minutes thing (the longest road tunnel is just over 15 miles long), but it also has no emergency exits other than the ends, and the maximum 7.6 mile trek to an exit with both ends clear (and, a fortiori, the longer possible exit journey if not starting at the middle but a near end exit is not possible) are likely to challenge people with small children. LLP
When I need to get from A to B, I just want to be there quickly. I can enjoy the surface of the Earth on my own time.
I'm sure there will be a need that arises for more tunnels for people, too. But I think the place to START and allocate resources is tunnels for stuff since that's where the problem will come from.
There are plenty of reasons to move people underground rather than above, along with goods. My personal interest in this is that a scalable 100% underground transportation system would make for a better model for life on the moon or Mars than anything we currently have on Earth.
Besides, traveling underground doesn’t have to be a dreary, dystopian experience. If we moved people in rectangular structures resembling elevators, we could cover the floor and ceilings with LCD screens, so instead of feeling like you’re inside an elevator, you could set the screens to show a panorama of the city you’re in, or the Swiss Alps, or if you throw a camera and microphone in there, you could attend a meeting while you’re on your way to that meeting.
Even if we get goods moving off the roads, personal transportation still has enormous costs. Just ask any American who has had to choose between paying high rent to live close to work, or buying a car to commute further away from their lower-rent home.