BBC News Online's Kevin Anderson in Washington

Airline passengers in the US braced for delays as new security measures were implemented as a result of the 11 September attacks.

But, while some airports experienced congestion, long queues thinned as the day went on.

And Americans said they are ready to accept some inconvenience in exchange for improved security, which they felt was absolutely necessary in the light of continued terrorist threats.

Washington's Ronald Reagan National Airport has security concerns and precautions unique from any other airport in the US.

The airport was closed for longer than any other after the attacks because of its proximity to the Capitol, the White House, and military installations including the Pentagon.

Reagan National's proximity to the US capital has prompted unique security measures

It still is not operating at the volume it did before the attacks.

Passengers are required to stay in their seats for the first 30 minutes after takeoff and for the last 30 minutes before landing at Reagan National, or the flight will immediately divert to Dulles Airport on the outskirts of Washington.

And passengers on the frequent shuttle flights between Washington and New York must stay in their seats for the duration of the less-than-an-hour flight.

Armed air marshals are also on many of the flights to and from Reagan National.

But Reagan National, like all airports in the US, faced an 18 January deadline to institute the new post-11 September security measures.

These require all luggage to be screened using one of four methods. Bags must be screened by electronic bomb detection machines, by hand, by bomb-sniffing dogs or by ensuring that all bags on a flight are matched to a passenger.

Flagged for suspicion

David Kiracofe is a pilot for a charter company. The clean-cut 21-year veteran of the US Air Force would not on first glance appear to be someone who would be flagged as a security threat.

But to get to the city where he has to fly from, his company often buys him one-way tickets on commercial airlines.

That is enough to raise suspicion. He has been pulled aside for additional screening since the attacks last September, so now he always travels in his pilot's uniform, never in civilian clothes.

Flying in and out of several airports, he said that security measures and delays vary.

Flying out of Las Vegas, he once stood in line for three hours just to reach the check-in desk, and he still faced long queues at security screening. He missed his flight.

Americans are willing to brave security queues if it means greater safety

But he said that in St Louis, the airport had a system that prioritised passengers based on their departure times and waiting times were minimal.

He is unsure whether the new security measures will result in better security. He said that only time would tell.

The new security measures probably won't have any impact on the infrequent flier who takes only a flight or two a year, he said.

But he feels sorry for business travellers who must fly often. "Suddenly, their lives are a pain in the ass," he said.

Safety primary concern

Passengers at Reagan National were ready to accept some delays due to increased security.

Todd Smith was standing in line to have his baggage screened by one of the new electronic bomb detection machines.

He has noticed increased security since the attacks. "You will see well-armed troops throughout airports, particularly at gates leading (to flights) to and from National Airport," he said.

"The hassles are to be expected. I can accept them. I've gotten where I needed to go safely, and that's what it's all about," said Mr Smith.

Angela Kim usually flies out of Baltimore-Washington International (BWI) Airport 45 minutes north of Washington, and she said that security there is very tight.

Two months ago, she arrived four hours before her departure out of BWI to make sure she could make her flight. "The security line was so long," she said.

But waiting for security at Reagan National, she said the queues were not bad and said the tighter security was necessary.

"We need to have tight security, check everything," she said, adding, "I can suffer the inconvenience for the security of myself and our nation."