Compared to Europeans, Americans are relatively infrequent users of public transit. In a recent report on transit systems in various U. S. cities, researchers from the University of Minnesota's Center for Transportation Studies attempted to address this issue. Although underuse of public transportation is a complex problem with many contributing factors, the report's authors noted one significant factor: when cities limit how tall their buildings can be, they tend to end up with more of what urban planners call . Sprawl is the uncontrolled expansion of a city's population away from an urban center into suburban areas. Sprawl makes public transit impractical for many residents because they must travel long distances to and from many different points. The authors recommend that cities pass laws to encourage people and businesses to cluster together in centralized areas. The idea is that the resulting high-density areas will be easier to serve with public transit.