LARGE, FAST-GROWING URBAN CENTERS that look nothing like traditional cities are suddenly bursting onto the urban scene, forcing planners and policy makers to take notice. These accidental cities, defined by a new Fannie Mae Foundation report as “boomburbs,” are growing rapidly and posing challenges to the city-building profession.

According to the authors of the report, Robert E. Lang, director of urban research for the Fannie Mae Foundation, and Jennifer LeFurgy, director of housing demographics, boomburbs are defined by their high rate of population growth, large size, and lack of a dense business core. They also differ from older suburban cities in their racial and class diversity. Most are affluent, but many also contain lower-income neighborhoods. They are often organized into master-planned communities whose homeowner associations act as small private governments to deliver services to residents.

Because they are based on sprawling, low-density development, most boomburbs are reaching a stage called “build-out.” These accidental cities may need to rethink their land use patterns and move toward denser urban forms in order to continue growing and avoid being left behind by their faster-growing older neighbor.

Many older boomburbs have a downtown, but it’s often not well integrated into the rest of the city. The downtown may be an old main street, a redeveloped historic center, or a brand-new center built in a greenfield setting. A revitalized downtown can help a boomburb retain its identity, and attract residents and visitors from its surrounding suburbs.