Could suburban sprawl be good for segregation? Duke Today, September 23, 2014. Racially and economically mixed cities are more likely to stay integrated if the density of households stays low, finds a new analysis of a now-famous model of segregation. By simulating the movement of families between neighborhoods in a virtual “city,” Duke University mathematicians show that cities are more likely to become segregated along racial, ethnic or other lines when the proportion of occupied sites rises above a certain critical threshold — as low as 25 percent, regardless of the identity of the people moving in.