The skies will probably be less than friendly Friday as strict security measures kick in at airports across America.

Bags and suitcases will be scrutinized, papers pored over and, at Continental Airlines at least, no luggage accepted within 30 minutes before flight time. Customers are being urged to arrive at the airport two hours early.

It's enough to make passengers wish flight attendants handed out aspirin along with the honey-roasted peanuts.

But flight experts were optimistic.

"Hope for the best, plan for the worst," advised David Stempler, president of the Air Travelers Association, an advocacy group. "And I don't think the worst is going to be so bad. We're not predicting the huge lines we were worried about."

Friday was the legal deadline for security provisions mandating that all bags have to be checked for dangerous materials, whether by machines, sniffer dogs or by the human hand.

And because there aren't enough explosive-detection devices, airlines also planned to match baggage with passengers so that no bag would be loaded that isn't accompanied by a passenger.

Carry-on luggage, already inspected at check-in, is not affected by the new security changes.

It's hard to judge how bad it will be, airline-security experts said.

"We're very hopeful we will not see chaos," said Carol Hallett, president of the Air Transport Association, the trade group for the major airlines.

The Bush administration asked passengers to be patient.

"I'm not sure that anyone really has a crystal ball that can determine how long those delays will be, but I think today passengers are willing to accept a modicum of inconvenience given the safety and security of the air travel," Transportation Secretary Norman Y. Mineta said on CBS' The Early Show. "And I think in today's world, patience is a new form of patriotism."

Stempler said it wasn't as bad as it could have been because airlines only need to match bags with a passenger on the first leg of a flight. A second match is not required if the passenger connects to a new plane.

That provision was denounced by Democrats in Congress, among others, with Rep. Jim Oberstar of Minnesota, top Democrat on the House Transportation Committee, saying it amounts to "an Achilles' heel in the security system."

There may be other delays at boarding gates, however. If a passenger fails to show up after checking luggage, a plane's takeoff might be postponed while airline employees remove the bag from the cargo hold.

"It's not unusual to be delayed 20 or 30 minutes for a variety of reasons," Stempler said. "I don't think passengers will experience anything extraordinary."

The Associated Press contributed to this report.