During the first 15 years of the century, Americans flocked to cities. Today suburbs are the hot new destination. According to Census data, the top 10 fastest-growing counties last year were in the South and West of the U.S. The Dallas/Fort Worth area added the most people, 146,000, three times as many as New York. A few drivers of the trend: More baby boomers are retiring in warm, low-density areas; high costs in cities keep would-be residents away; and as millennials have kids, they’re deciding that having a lawn might not be so bad.