The invention of the car caused suburbanization. As soon as cars were invented roads and gas stations started springing up. It's natural that people followed those.
-land rights mobility
-class system mobility
-economic mobility
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.
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.
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.