The obvious answer is more public transport infrastructure & bike lanes.
If you think population density is an excuse for public transport infrastructure not coping or need for more people owning cars I suggest taking a long hard look at e.g. Japan to have that hypothesis reality-checked.
I'm btw. not saying you did, just reading between the lines.
As I wrote in an earlier reply to parent, NYC hasn't managed to even build ring Metro lines around its city center – since a century!
And that is for one reason and one reason only: not nearly enough (political) pressure from the public to improve public transport infrastructure.
And that in term gets us to the root cause again: the US can't imagine itself without cars.
This is not a critique. It's just an observation that is very plain to see if you grew up in Europe (and possibly many other places, too).
When/if that changes, ever, the above things will just happen naturally.
What would a ring metro look like in NYC? Manhattan is an island. Directly west of it is a body of water, then land that is not NYC, in fact is not NY.
It doesn't have to be a closed ring. Or resemble a ring.
Many metro systems in other big cities with comparable topological constraints have metro lines that are orthogonal to those going out in roughly a star pattern from the center.
But NYC almost only has the latter.
Underwater sections are not an issue, really. There are many cities that have metro lines going under bodies of water.
And even the length or depth required is not an issue if you want to build it.
That's why you can go from England to France, by train, in roughly half an hour, under the English Channel.
If you think population density is an excuse for public transport infrastructure not coping or need for more people owning cars I suggest taking a long hard look at e.g. Japan to have that hypothesis reality-checked.
I'm btw. not saying you did, just reading between the lines.
As I wrote in an earlier reply to parent, NYC hasn't managed to even build ring Metro lines around its city center – since a century!
And that is for one reason and one reason only: not nearly enough (political) pressure from the public to improve public transport infrastructure.
And that in term gets us to the root cause again: the US can't imagine itself without cars.
This is not a critique. It's just an observation that is very plain to see if you grew up in Europe (and possibly many other places, too).
When/if that changes, ever, the above things will just happen naturally.