> Unfortunately, the American population has also exponentially grown in that same time period.
Even if that were true (I'm pretty sure even with immigration, the doubling time of the US population has been increasing over that period, so the growth has not been exponential, which would be constant time to any given multiple) why would that particular shape of the growth curve make a difference?
If you are just misusing "exponential" to mean "fast", more population in the same area makes mass transit more, not less, viable an alternative to individual transit.
Was there a reason to be pedantic here? If you look at [1] then the US population has grown exponentially, and we are in the higher slope portion of the curve. But if you zoom into 1920-{current} you can form a linear curve.
Anyway, city planners are not looking at aggregate population statistics and making their decisions based off those. Usually, cities grow due to migration of new people. As more people enter the city, city governments decide what infrastructure to add in order to sustain population growth. For many years, the consensus was simply to add more roads. Studies eventually revealed the phenomenon of Induced Demand [2] which caused more roads to be consumed after creation of the roads. This and the realization that roads filled with single-occupancy vehicles don't scale well has led to a more "modern" consensus that transit makes sense once cities become big enough. For a long time, people thought that building more roads was the way to make a city more accessible.
Even if that were true (I'm pretty sure even with immigration, the doubling time of the US population has been increasing over that period, so the growth has not been exponential, which would be constant time to any given multiple) why would that particular shape of the growth curve make a difference?
If you are just misusing "exponential" to mean "fast", more population in the same area makes mass transit more, not less, viable an alternative to individual transit.