Would be a lot cheaper and more permanent to just have other modes of transportation.

Unfortunately just severely less useful in the majority of the US.

But very useful in the places where you'd get stuck in a traffic jam.

The majority of people in the US live in urban and suburban areas. There’s plenty we can do in these areas to reduce car-dependency instead of paving more roadway (which ends up as waste of time due to induced demand).

I think you misunderstand induced demand. The increased demand when you add more capacity, simply means even more utilisation is achieved -> increased productivity. This is not a waste of time.

The correct comparison should be comparing for each area and situation, which type of transport investment results in the greatest utility. The objective is not to minimise traffic, but to maximise peoples ability to get where they need to be.

No, it just makes things farther apart, and more people sit in traffic jams. That's decreased productivity.

By that logic: How can you actually increase utility for trains if you don't build a train? If public transportation is eschewed in favor of more roadways, then unsurprisingly the utility of the roads increases.

But, many studies have shown that building more roads does not reduce congestion. Congestion is not function of roadway space-- other factors like accidents, merging, exiting, peak travel capacity, etc greatly impact congestion. Building another lane on a highway doesn't solve the problem.

A common but misguided desire.

Of course, but it would be fun to design and build. And since it will obviously never happen it seems pretty harmless to think about?

As an engineering challenge, yeah, the machine itself is super interesting. But "more road" isn't the answer to road congestion so it's a machine without a purpose (unless the purpose is propping up the oil & auto industries?).

We need a lot more than one more lane. Traffic continues at high speeds long after it isn't safe. You are supposed to see 2 seconds between cars, yet when I get on a busy road and do that I often see 5 cars in the lane next to me in that distance. Little des moines where I live needs Huston levels of lanes to allow for safe travel. For a city with significant population you need a lot more.

Or of course better public transit. The vast majority of what people are driving for could be done without driving if transit was better.

car travel would be a lot less appealing in throughput if subject to the same stringent separation requirements as rail travel.

The stringent separation requirements are because of the long braking distances of trains, cars have much shorter braking distances - however even those are routinely violated by drivers. In Germany, the rule of thumb is "half your speed", i.e. 25 m for 50 km/h, 50 m for 100 km/h etc.

The US used to use a rule of thumb like that, but counting seconds is a lot easier than estimating distances.