One thing I don't get is why a tunnel? Why not a second level instead? That's what most highway focused urban areas do, like Taiwan, KL and China below.
Because its every bit as silly as the tunnel idea.
Common problem:
Insufficient capacity on interchange roads to handle more incoming or outgoing traffic.
Where are the ramps going?
Benefit: (arguably) avoid certain conditions associated with tunneling
Negative: Toronto's experience with the Gardiner indicates support piers/bents have a life expectancy below 50 years, reconstruction and ongoing maintenance in a road salt environment are hideously expensive.
Negative: Noise pollution Car traffic located above near-by housing will see noise spread far and wide, unless, of course, you want noise barriers 4 storeys high.......expensive and visual blight even before thinking about tagging.
Negative: You still need bridges over the major river valleys, but now they are bridges with piers that will straddle the existing bridges, unless you divert to the side, either way, very expensive and environmentally consequential.
*****
Up/down changes the how, but not the why/why not. The answer is still no, because it would actually make overall traffic worse than it is now, at best.........it simply cannot achieve the desired goal at any cost, let alone a reasonable one.