Not solving but alleviating, by rerouting transiting traffic through bypass routes underground. For example, in Washington DC:

https://en.wikipedia.org/wiki/9th_and_12th_Street_Expressway...

But does rerouting traffic through other bypasses actually alleviate traffic?

> Motorways and bypasses generate traffic, that is, produce extra traffic, partly by inducing people to travel who would not otherwise have done so by making the new route more convenient than the old, partly by people who go out of their direct route to enjoy the greater convenience of the new road, and partly by people who use the towns bypassed because they are more convenient for shopping and visits when through traffic has been removed

From https://en.wikipedia.org/wiki/Induced_demand

Do look at the "Studies" section of the same article, too.[1] It is far from established that induced demand outweighs the capacity increase from road construction.

[1] https://en.wikipedia.org/wiki/Induced_demand#Studies

Mayors of big towns are clearing city centers from cars and making them walkable again anyways (at least in Europe, I don’t know how it is being done in other places). If tunnels are the shortest path to get from one part of the city to another (like in Zurich), cars will use them.

A traffic network's purpose is to move people from point A to point B. If you double the lanes and they fill up, you've doubled the throughput. If traffic doesn't move any faster, it's because the old traffic jams were a bigger problem than was visible. You've still alleviated the issue by allowing twice as many people to reach their destination in the same amount of time.

There's an argument that assumes that building more lanes does not increase the number of people who travel, only the number of people who choose to do so by car. Then with fewer lanes, the same number of people still get from A to B but on bicycles or using public transport instead of cars.

These photos illustrate it well:

https://www.streetroots.org/news/2015/04/03/streets-are-ever...

Less cars on the road solves traffic jams. Not more roads. Traffic jams are insane, we see a single person sitting alone in a 3+ ton box idling his time and instead of seeing how absurd it is, we want to make bigger roads for more almost empty boxes. Nuts. The solution to traffic jams is excellent public transportation and last mile mobility. Not more roads and definitely not tunnels.