They will never, for good reason, be built for most routes.
Because they're very expensive, both to build and operate, so they're only sustainable between large cities at a mediums distances apart.
China, with maybe 10x the population density of the US, has a huge number of such routes. The US does not.
https://en.wikipedia.org/wiki/List_of_cities_in_China_by_pop...
In fact, the cost of building & maintaining the highway system is quite astronomic.
If we were designing the interstate system today, that's actually a fun thought experiment. What would we change? I would probably focus more on throughput -- cars can be half as wide and still seat tons of people, 5 lane highways today could easy be 8 lanes of narrower cars.
Peak hours are always going to cause congestion.
As you point out, several studies have shown that over time, travel time converges to the same value, despite additional capacity.
However, I think it is important to note that more lanes means more people are still getting from point A to B.
The only problem is we can't afford to build enough lanes. There is a limit to how wide you can go, and up or down is vastly more expensive than at grade. We could do it if we wanted to spend the money - but most such places find that a better bus and (not or!) train network is a better investment.
Why would you go narrower? That makes problems worse for trucks and other cargo conveyance; note that in general, newer highway standards tends to increase lane width, not decrease them compared to older standards.
(I guess you can see the same effect with railroads--the UK railroads have smaller loading gauges than newer networks like the US, Germany, or Sweden).
If it takes only marginally more, or even less time to travel by train than by plane, I'm taking the train any day.
And I'm talking above 120mph. None of this 70mph "high speed" slow stuff.
If you've got real numbers, I'd like to see them.
How do you secure a railway?
If trains go 600 kph, do we get a TSA circus along the length of all railtracks? Is this even feasible?
"hundreds occupied high-speed railway tracks for around 3 hours, blocking services linking Figueres, Girona and Barcelona,"
https://www.thelocal.es/20181001/pro-independence-protesters...
I once rode the Eurostar from London to Paris and as I recall all the high speed sections were enclosed in very high, barbed wire fence. So some security is definitely needed. Collision energy goes up with the square of velocity.
Spain has plenty of them, according to Wikipedia only China has more. And if I'm not wrong, Spain is building them in other countries too.
https://en.wikipedia.org/wiki/AVE
https://www.pbs.org/wnet/blueprintamerica/video/web-exclusiv...
I see that mostly as part of the larger problem that the US doesn't build much of anything these days.
The US has several population clusters[1] and for example the Northeast corridor in particular with 50 million inhabitants is a reasonably good target for a high speed rail network.
[1]https://en.wikipedia.org/wiki/Northeast_megalopolis#/media/F...