Well, that's different, related, and very simple. If it's more convenient to not use a car at home, (especially if there is never adequate parking where consumers need it,) people will be far less likely to use them while traveling.
Also, have the National Parks Service severely restrict Automobiles in National Parks, to prevent pollution, and damage to the environment (People are idiots and cars of the era were very dirty, so it makes a hell of a lot of sense, especially if very few people actually drive in the parks to begin with, and keep hitting things.)
And yet even in the 20s - before the successful attempt to destroy light rail - we see automobiles as convenient enough for this.
Also:
Even if you restrict automobiles IN National Parks, that doesn't rule out using them to GET to National Parks, which is where the Yosemite example comes in.
By the way, since the issue of numbers of Americans with cars came up.
After checking the census (All figures rounded for convenience):
106 million (approximately) Americans.
33 million males over 21. Assuming an equal distribution of males and females for calculation's sake (I can probably find the figure if I try), that's about 65 million Americans over 21.
Obviously there wasn't an age requirement to get a driver's license in this era, but since someone under 21 probably can't afford even a cheap car on their own, it seems fair.
That means if one in ten Americans own a car, and assuming each American owns only one car, there are ten million cars on the road.
Approximately one car for every six adult Americans.
It's not yet "a car for everyone", but its too late for it to be a rich man's toy - the mass market has been found and is being exploited.
And what's the incentive for the developers to build streetcar suburbs to such an extent as to dominate the urban landscape?
I don't think you can keep the car from being, at the very least, popular among the middle class - the working man might take the streetcar and not have the money to "splurge" on a new car, but even that seems unlikely to be completely successful as a way to keep the car from becoming popular - slow things down, yes, eliminate it, no.