Cars were widespread and affordable in the 20s and 30s. Car dependent suburbs without street cars didn't happen until the 40s and 50s. It's not as simple as just cars. It was the confluence of Eisenhower's interstate program and white flight from blacks as well as a crescendo of corruption and incompetence in the governance of the urban cores.
I think a major piece of the puzzle is that living in a car dependent area has been an effective way for the middle class to segregate from poor people. If you're not rich enough for the well policed part of the city, you can still get a car and live somewhere poor people have difficulty getting getting to.
I would suggest that saying anything as substantial as an automobile being "widespread" or "affordable" during the 20s and 30s is overreaching, considering there was something of a worldwide economic instability at the time.
I think a major piece of the puzzle is that living in a car dependent area has been an effective way for the middle class to segregate from poor people.
For me, personally, this very much drives my choice to live in the suburbs and to drive a car instead of using public transportation: I'm unwilling to settle for the lowest common denominator.
My point was the highways were an indirect effect of cars. Highways were a secondary effect of cars and suburbs were a tertiary effect of cars.
What you say makes sense but I think suburbs would have sprung up over time without highways just because the population was growing and people would be willing to move further away from urban areas as cars became safer and more common.
Google "street car conspiracy". The evidence is that car ownership rates had pretty much leveled off. Everyone who wanted a car had one. Most people were pretty happy with street cars. There was an active campaign from moneyed interests to use government to build highways and rip up street cars.
I think a major piece of the puzzle is that living in a car dependent area has been an effective way for the middle class to segregate from poor people. If you're not rich enough for the well policed part of the city, you can still get a car and live somewhere poor people have difficulty getting getting to.