Last summer, the Michigan town of Grosse Pointe Park erected a farmer's market in the middle of one of the few remaining streets that allowed cars to pass between the tony suburb and the urban Detroit neighborhoods at its border. It was the latest of many attempts by Grosse Pointe Park residents to close off roads and block traffic between what has become a predominantly white, affluent suburb, and its poorer, urban neighbor.

There were protests about the border, and Grosse Pointe Park later said it would tear down the farmer's market and re-open the road, but the incident speaks volumes to the segregation that exists in Detroit, and the tensions that can grow as a result.

The fact that these two areas are so close is unique—the border between Grosse Pointe Park and the city of Detroit is the only place in any of America's biggest cities where a very wealthy, predominantly-white area abuts a very poor, black one, according to research from a new working paper from the University of Minnesota. But the existence of self-segregated wealthy white areas close by low-income minority ones isn't unique, according to the Minnesota researchers. They have sorted census tracts in 15 of America's 20 biggest cities into "racially concentrated areas of affluence" and "racially concentrated areas of poverty," and find that many cities have more areas of segregated affluence than they do poverty.

Racially concentrated areas of affluence, by the researchers' definition, are census tracts where 90 percent or more of the population is white and the median income is at least four times the federal poverty level, adjusted for the cost of living in each city. Racially concentrated areas of poverty, by contrast, are census tracts where more than 50 percent of the population is non-white, and more than 40 percent live in poverty.