So, first: understanding segregation law with more sophistication requires that we set aside "racism and bigotry" as the sole motivation for such laws. One way that you might begin to answer this question, then, is to focus on segregation in the twentieth-century North/Midwest. By focusing on segregation outside the nineteenth-century South, we might gain a different perspective that positions segregation in relation to a number of beliefs, particularly residential rights, local governance, and workers' rights, as well as the growth of American real estate markets and the "origins of the urban crisis" (to quote Tom Sugrue).

For instance, until the Fair Housing Act of 1968, the federal government subsidized home loans on the basis of housing stock and demographics (calculating loan risks from "security maps") in a process known as "redlining" (based on the colors used in the security maps). More precisely, the federal government would not loan to home owners if their neighborhood was racially mixed, or likely to integrate. The result, then, was that the federal government actively promoted segregated neighborhoods. This practice has, according to Tom Sugrue in Origins of the Urban Crisis and others, set up American cities to fail in the manner of Detroit by geographically setting in place pools of capital and pools of poverty.

In another way, the federal government also upheld segregation on the basis of home owners' prerogatives. Prior to the SCOTUS ruling in Shelley v. Kraemer in 1948, housing covenants could restrict potential residents on the basis of religion, race, etc. For the proponents of housing covenants, the home owners' prerogative to self-segregate rested, in part, on Americans' constitutional rights to property ownership.

Beliefs in the sanctity of property ownership also aligned with other political concerns, such as happened (and continues to happen) throughout New Jersey. According to Liz Cohen in A Consumers' Republic, New Jersey has erected a vastly unequal school system that privileges white suburban schools to the detriment of largely black urban schools (both inadvertently and intentionally) by the ways that counties use local taxes to fund schools, rather than distributing funds through the state government. New Jerseyites' (?) attachment to localism has helped to entrench segregation without the need for explicitly privileging racial categories.

Finally, you might think of segregated labor forces, and workers' prerogatives as both union members and employees. In 1941 and 1943, white Detroit UAW members struck to keep black members (recently from the South) out of their shops and their unions. In part, shop owners realized that hiring unskilled recent black migrants might be an effective tool to lower wages and break the back of the recently organized UAW shops. None of this is to say that workers' had no pre-existing prejudices, but, that anti-black racism could easily align with the interests of workers and employers alike.

The question I take it concerns the Jim Crow laws adopted throughout the South in the late nineteenth century, after the collapse of Reconstruction, and rigorously enforced until they were gradually dismantled following the Supreme Court's decision in Brown v. Board of Education (1954). Racism was common in the North as well, but legal segregation in the South approached the virulence of Apartheid in South Africa, and was enforced with lynch law. Millions of people fled to the north. The minority white governments of the South justified the segregation laws by asserting that they did no more than separate the warring races, in public places, in order to keep the peace. The Supreme Court accepted that rationale in the infamous case of Plessy v. Fergusson, 163 U.S. 537 (1896), in which Justice Harlan issued a famous dissent saying correctly that the purpose of the laws was to recreate the subordinated caste system of slavery. In the twentieth century the Jim Crow regime was expanded to encompass every aspect of civil society, and the rationale was the fear of race mixing, miscegenation, and fearful fantasies of black sexual predators. In 1917, in Buchanan v. Waverly, the Supreme Court rejected the race-mixing argument, but this and sexual fears continued to be offered to justify the separation of the races, carried out by private measures enforced or ignored by the states.