At least in major Southern cities, the problem was often "the blacks." Highway locations were often chosen to cut off less-desirable neighborhoods from city centers and more upscale residential neighborhoods. (I-30 in Dallas and the proposed I-485 in Atlanta, and arguably Robert Moses' Cross Bronx Expressway all come to mind.)

Today, the need for highways hasn't gone away, but more enlightened urban planning and urban gentrification means that cities are finding ways to either relocate or ameliorate the effects of those roads -- though the expense is significant.