This technical-fix piece begins like so many others in approaching the question from a solution-first perspective ("put roads underground"), without considering what the problem being addressed or goals sought are.[1]

If the goal is to reduce street traffic, then ultimately:

- Needs for trips should be reduced.

- Travel for essential trips should be minimised.

- Longer-distance travel, both intra-city and inter-city, should be optimised for mass transit to the greatest extent possible.

The section of Chinese expressway shown here has 23 lanes, and shows about 72 automobiles in each, for a total of about 1,700 cars.

http://www.zo.utexas.edu/courses/THOC/

Assuming occupancy of 1.4 persons per vehicle, all the passengers shown could be accommodated in 3 typical Chinese high-speed rail 16-car trainsets at 50 passengers/car (well below full capacity), probably double-tracked.[2]

The theoretical maximum highway capacity is 2,200 vehicles/hour per lane. A 16-car trainset at 100 passengers/car can accomodate 1,600 passengers. These can run at headways of 4 minutes, or 15 trains per hour, for a net track capacity of 24,000 passengers/hour, more than 10 times that of a single highway lane. The 23 lanes shown here could have an equivalent capacity from four rail lines, assuming the highways are flowing freely (which in all likelihood they are not).[3]

In practice, both rail and passenger-vehicle capacities would be reduced, though rail should still outperform private automobiles by a very wide margin.

What automobiles did was to reduce the cost of space and disatance. The result was that cities and metropolitan areas sprawled. By increasing costs, deprecating automobiles, and supporting local-first, walkable, bikeable, and transit-centric development, traffic, congestion, noise, pollution, and fuel/energy use would all be greatly reduced.

I suspect there's something of a Gresham's Law of urban planning in which even accommodating automobile traffice tends to drive out other modes (walking, cycling, transit) through both space use and travel patterns which emerge where automobiles are an option. See also Alfred E. Kahn's "The Tyranny of Small Decisions".[4]

Undergrounding roads is fantastically expensive, introduces numerous problems (congestion, air quality, fires, flooding, roof collapses (as experienced on Boston's Big Dig), and more. That's not to say it's never an option, but it's well down the tools list I'd suggest employing.

________________________________

Notes:

1. I'd come up with a hierarcyy of failure in problem resolution a few years back, or if you prefer a positive spin, reverse the senses ("failed" becomes "necessary" steps), to come up with a success-process chain: https://old.reddit.com/r/dredmorbius/comments/2fsr0g/hierarc...

2. For typical Chinese passenger rolling stock: https://en.wikipedia.org/wiki/China_Railway_CRH2

3. Traffic capacities for various road and load profiles: https://www.engineeringenotes.com/transportation-engineering...

4. https://en.wikipedia.org/wiki/Tyranny_of_small_decisions https://onlinelibrary.wiley.com/doi/10.1111/j.1467-6435.1966... https://libgen.rs/scimag/10.1111%2Fj.1467-6435.1966.tb02491....