14% of Americans are black. Without debating that the US is full of systemic racism: Do you really think anybody believed "well, with 14% less cars, this works"? And this isn't even accounting for the fact that white people buy significantly more cars.
We can add that 34% of black people use public transit, and only 14% of white people do. The problem really isn't black people having cars, or the missing assumption of black people having cars.