Low-fare airlines now account for 24 percent of domestic scheduled airline seats.
AirTran has been the most aggressive, starting 13 non-stop routes in the past year. They have connected cities as small as Akron, Ohio, and Moline, Ill., that previously lacked low-fare service.
By contrast, the largest low-fare airline, Southwest, has started fewer than six routes in the past year, mostly long hauls, such as Baltimore-San Jose, Calif.
Newest low-fare service is going to Canada, Mexico and the Sun Belt -- about half of the new routes connect California, Florida, Nevada or Arizona to other states.
"It's getting a little more crowded for the high-cost airlines," Tom Parsons, publisher of BestFares.com, told USA Today.
It's getting crowded for the discounters, too. The heated competition has driven some low-cost competitors, such as National and Vanguard, out of the skies.