Race was a huge motivator. Why else would we have racial covenants? Funny how the growth of suburbs came at roughly the same time that schools started integrating and integration accelerated suburban growth. We had Federal suburban housing subsidies available almost exclusively for whites.
We have a name for this phenomenon: White Flight.
Pre-1950s we had racial covenants, but we didn't social engineer lifestyles in suburbs to build them around cars. Streetcar suburbs for instance looked similar to the cities they were connected to. Smaller and less dense, but walkable urban places in their own right. Look at places like Del Ray, Alexandria, Virginia for an example.
In cities themselves we tended to build highways in poor, mostly minority, neighborhoods. The white neighborhoods wouldn't be leveled nearly as often.