Isn't this similar to adding lanes to freeways? I was always under the impression that actually makes traffic worse, like this article suggests: https://www.wired.com/2014/06/wuwt-traffic-induced-demand/

We can let traffic get so bad it reaches an equilibrium that stiffles the growth of the region by preventing movement, or continue to expand capacity indefinitely.

It's more like adding a new road. You can't "change lanes" between the tunnel and the freeway.

I'm a complete novice when it comes to road infrastructure so I really have no idea. But I always thought the argument was expanding road capacity just caused more people to consider using it and it always naturally expanded to the point of congestion regardless of size. I'd think a road plus a tunnel would have a similar effect.

have you ever been in a traffic accident on the freeway? It blocks up every lane. Whereas a completely different route can still proceed unaltered. Also, splitting up the traffic should theoretically reduce accidents.

Sure, but in my experience accidents are very rare compared to the total. I don't live in LA, and my evidence is anecdotal, but I'd be willing to bet that overall accidents have minimal impact on a road's throughput.