They absolutely will not. What kind of weird 'thought experiment' is that?
Well......I mean they certainly aren't going to catch up to us (Canada) proportionately anytime soon..........but there are ~14 Metro/Subway expansions or new lines currently under construction in the U.S. and several more quasi-serious proposals, along with 138km of light rail/streetcar.
But you're not wrong here. I'm just adding some colour.
The thing is,
@Tim MacDonald is completely correct with his 'nothing' in terms of the impact at the current level or even if you doubled, tripled or quadrupled it.
There are multiple reasons for this, including underfunded service........lets look at Dallas' system for an example, peak service on a weekday in rush hour for their Blue Line is every 15M.
The Green is the same; off-peak mid-day is every 20M, while mid-evening........for a subway/LRT is every 28M.
At that level of service..........or even at double that level of service you're just not moving the needle very far.
Certainly not far enough to affect car ownership levels, on a national scale.
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Additional examples:
MARTA (Atlanta) has rush hour trains at every 10M on its Red Line, Every 12M in mid-day, Oh, and last train is 8:50pm
Its Gold Line runs a full schedule at the same levels as above, but every 20 minutes in the evening.
SEPTA, Philadelphia - Every 6 minute peak and mid-day service on the Market-Frankford Line but every 12 minutes in the evening
Its Broadway line is every 7 in peak/mid-day and every 12 in the evening.
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Additionally you have all the highways, the auto-centric development patterns etc etc. which may well evolve, but not all that quickly.
Suffice to say...............the kind of change that would see U.S. personal vehicle ownership slide.............is a very, very long way away.