Early this month, Continental had 676 self-service check-in machines in 113 airports in the United States.

American is continuing to install self-service check-in machines nationwide, with check-in machines for e-ticketed passengers in 39 airports as of last month.

Self-service check-in machines, once an oddity, are becoming almost as ubiquitous as automated teller machines and often outnumber agents at some airlines' ticket counters.

In addition to the self-service check-in machines at airports, Delta, Northwest, American, Alaska and US Airways also allow passengers to check in online and to print boarding passes from a home or office computer, in some cases up to 30 hours before a flight.

Delta plans to spend up to $30 million this year to redesign its airport lobbies, adding more than 400 self-service check-in machines to the 400 it now has and reassigning customer service agents to roles directing passengers to the fastest check-in alternative and helping those who need additional assistance.