Is it true that many American cities built highways as a way of dividing white and black neighborhoods? How was this done?

I have not ever found any evidence of this being done deliberately, though the story is a bit complicated because a lot of freeway construction coincided with the height of slum clearance and urban renewal programs.

Initial plans for superhighways were drawn up in many big cities in the 1930s, long before the 90 percent federal funding that would appear in the 1956 federal highway bill. Pressure to minimize costs was always present, but was especially vital prior to the federal funding. Largely for that reason, but also aware that the new superhighways would divide communities, planners tried wherever possible to route the highways next to existing railroad lines or waterways, and sometimes in or next to large parks. Such a corridor wasn’t always available, and the relative costs of various alignments had to be considered. Often the least costly alignment was through obsolete warehouse and manufacturing areas, through declining areas of substandard housing, or areas where street grids collided (reducing the number of needed overpasses).

After World War II, when many states and cities were finally able to begin superhighway construction in earnest, other programs to revive decaying cities had been put in place: slum clearance and urban renewal efforts. Judged by today’s standards, these programs had their own problems, but at the time they seemed the only realistic solution to urban blight. In the mid 1950s, as Congress tried to work out the financing of freeways, big-city mayors and congressmen also demanded that the new network serve rather than bypass their inner-city industrial and port areas. (As intercity trucking grew during the 1930s to 1950s, suburban industrial areas with cheaper land and more room for storage had become more attractive for factories.) General routes for the network, including urban segments, had been set out in the 1955 Yellow Book but those weren’t specific enough to be identified with racial barriers or particular neighborhoods. However, by the time Congress finally arrived at a solution to financing the nation’s superhighway program in the 1956 Federal-Aid Highway Bill, cities sometimes found that freeways could be built on newly cleared land rather than require additional buyouts of healthy, high-value property. Because those cleared blighted areas were often occupied by African-Americans, there was some truth in the cynical observation that “urban renewal means Negro removal.”

The claim that freeways were deliberately placed to divide the races is most frequently heard about Chicago’s Dan Ryan Expressway south of the Loop. The Dan Ryan route was set in 1939, and modified slightly in 1956. It was this modification that Mike Royko claimed (without attribution or citation) was racially motivated, in Boss, his biography of Chicago Mayor Richard J. Daley. To avoid taking homes, the only realistic routing options through Chicago’s South Side were either through industrial areas next to the C&WI railroad, or next to the Rock Island railroad. Here's a map I made to illustrate. The original 1939 plan (blue dots) used first the C&WI and then swerved over to the Rock Island. The 1956 change put the expressway next to the Rock Island the whole way, for the official reasons of “better alignment and traffic distribution.”

It’s hard to see how the new route divided the white ethnic Bridgeport neighborhood from the Black Belt any more efficiently than the original route. A significant number of blacks already lived west of the Ryan by 1956. Instead, as Dominic Pacyga's book Chicago: A Biography, notes (p. 337), Daley's motivation was not so much racism as protecting his political base in Bridgeport. The housing in the Black Belt had already been torn down for urban renewal. Why tear down perfectly good homes in Bridgeport in addition? The new route was also more direct, as can easily be seen. Traffic engineering may well have played the primary role, exactly as was said at the time. The routing in 1939 had been influenced by the cost of industrial property along the NYC, but by 1956 that situation had changed.

I like Earl Swift's The Big Roads as the most readable history of American superhighways, but Mark Rose's Interstate: Express Highway Politics 1939-1989 is the more complete scholarly source regarding the Congressional history. A more recent book by Joseph DiMento and Cliff Ellis, Changing Lanes: Visions and Histories of Urban Freeways, recounts the history of how traffic engineers and city planners wrangled with this new force reshaping the city.