When Brittney Ross moved to Summerville, S.C., last fall, she quickly realized she was part of a trend. Reminders of the state’s burgeoning population surround her: She and her boyfriend snagged an apartment in a newly constructed building, and she battles fierce traffic to commute to work. One bad day, her ordinarily 35-minute drive took more than two hours. Lines back up at her local Walmart, turning simple errands into unpredictable quests.