Fuller flights are a boon to airlines but overcrowding at gates has become an unwelcome side effect.

United Airlines says its customers have complained about clusters of travelers around its gates. Travelers spilled into corridors and arriving passengers who just stepped off a flight were met with hordes of departing travelers.

"It's too congested," said Maria Walter, United Airlines' managing director of global operations strategy. "It created a lot of angst from our customers."

The airline on Tuesday debuted a new boarding process in an effort to combat this passenger pain point, and make sure flights depart on time as more travelers than ever opt to fly.

United reduced the boarding lines to two from five, and is urging travelers to wait until their boarding group — numbered 1 through 5 — is called. United carried close to 155 million passengers in the 12 months ended in August, up 6.1 percent from the year-earlier period, the company said last week.

The airline now sends out notifications to passengers on their mobile devices to let them now boarding has begun once the first boarding pass is scanned, so that travelers don't have to hover around the gate.

Travelers would sometimes queue up an hour before a flight, Walter said, which would attract even more passengers to the line. "It's like a magnet," she said.